PASADENA, Calif. -- Martin Sheen is philosophical about The West Wing's leaving office tonight as one of the most venerable, and venerated, ensemble dramas in the history of television.

The time has come, Sheen feels, to call it a day.

In private conversation, he is cheerful and talkative -- he has aged physically, but he feels like a youngster again. The laughs come easily. Despite The West Wing's reputation for earnestness and his own, hard-won reputation as a firebrand political activist, he's quick to laugh at himself.

"I was constitutionally directed toward the exit," Sheen said wryly, about his own impending departure from the show. "I knew it was coming."

He thought until recently that Jimmy Smits' Matt Santos and Alan Alda's Arnold Vinick would pick up where the Bartlet administration left off, but veteran cast member John Spencer's sudden death in December hastened the end of the series, Sheen believes.

"It's probably the best way to go out. I think John's death signaled that it was time to leave, for all of us. We were the parents. We were the oldest guys. We had been around the longest and we lost one of us. It's hard to go on without a family."

As so often happens to actual U.S. presidents, Sheen has aged noticeably since that day in September 1999 when The West Wing aired for the first time, and a national television audience was introduced to President Josiah "Jed" Bartlet.

Nothing about The West Wing unfolded as it was supposed to.

Media analysts insisted that a mass audience would never embrace a talky ensemble drama about politics, especially one that presented public service as a noble calling, driven by idealists.

Sheen, a career actor who now counts The West Wing's Bartlet as one of his three defining roles -- the others are Captain Willard in the 1979 Francis Coppola war classic Apocalypse Now, and spree killer Kit Carruthers in Terrence Malick's 1973 social allegory Badlands -- was supposed to appear as a peripheral character only. The West Wing was ostensibly about policy wonks, overworked White House staffers and neurotic speechwriters, and it was all that. From that first scene where Rob Lowe's Sam Seaborn gets a page from the White House, though, it was clear to West Wing creator Aaron Sorkin and executive producer John Wells that everything that happened in the program, from the policy directives to the backroom deals to the rapid-fire discourse in the corridors of power, flowed down from the man at the top: Sheen's President Bartlet.

Sheen, 65, thought he had signed on for a part-time gig. Instead, The West Wing became all-consuming.

For four years -- Sorkin's period of tenure with the show -- Bartlet, and Sheen, became The West Wing's most readily identifiable face, its heart and soul.

Bartlet's screen time was whittled down in later years, but Sheen said that had nothing to do with the network's nervousness about his political activism.

"I wasn't a kid anymore, and I needed the three-day weekends. The amount of work and the hours were overwhelming, and I found my energy depleted. I needed more time. So that was given to me in lieu of more money -- which was far more valuable to me, frankly."

The years have not softened his activist leanings, but he is not about to throw himself back into the protest movement any time soon.

"I plan to do what is necessary," Sheen said, suddenly pensive, "and I hope I'm guided by a true spirit. I don't know when I'm going to be called, and to do what. I just hope I have the courage to speak truth to power, to do justice, to walk humbly and to serve."

He's not looking to land that one great role, either. Instead, he is going to earn the degree he never had at the National University of Ireland in Galway. He hopes to study English literature and philosophy, "and maybe theology, and maybe oceanography. ... An old guy like me has to start somewhere."

Sheen says only a fool would think a television actor would make a good president -- he has ruled himself out as a candidate for political office -- but he holds strong feelings about leadership just the same.

"I have a great respect for anyone who goes through (public office), no matter what their age, because it'll make a man out of you. I can't even imagine going through the real thing -- the campaign, the job itself, the demands, the time away from family and friends. It is a real, deeply personal sacrifice, and it's very costly.

"Leadership is example. Leadership is taking risks, being more fully human, not being afraid to fail. You have to go above your constituency. You have to go to the future. There's the old Hebrew adage: 'He that hath offspring giveth up hostages to the future.' If you are a leader, the best thing you can do is to inspire people to be better than they are, to say, 'We're better than this.' If The West Wing did anything, as a TV show, it was that it provided a sense of hope, a sense that we're better than this, that there are better days ahead."

Great leaders are rarely recognized in their own time, Sheen says.

"I hear Eisenhower now called a great president. When I was a kid, he was called everything under the sun, but not that. He was not appreciated until he was gone."

He hasn't seen ABC's recently cancelled Commander in Chief, although he wished Geena Davis well in person at the outset.

"It's a tough sell, politics (and TV). I thought it was great that there was a woman president. I had hoped that we would have one in my lifetime, and I still hope that we'll have one. There are so many women who are qualified -- who are overqualified -- for that office, and I hope to God we have that grace descend on us before long. Dostoevsky said that the world would be saved by beauty. I think he meant women."

For now, Sheen gets through life day by day.

"Every day is a risk and a blessing, of equal measure. ... You give thanks and praise, and hope you get through it."

Wandering in the West Wing

Sure, there have been a few of the seven seasons of The West Wing that didn't pass muster, when people were kidnapped or shot for no apparent reason. But most of the time, it has set the bar high and then climbed higher.

Memories? We've got a few.

-- Early on in the series, a young man watches the staff scramble around the Oval Office as the president prepares to deliver an important speech to the American people. The double jolt of snagging a White House job and this new and thrilling proximity to power leaves him reeling. "I've never felt like this before," he breathes. "It doesn't go away," replies the not-so hardened staffer next to him.

-- After the funeral for the president's trusty secretary Mrs. Landingham, filmed at the Washington National Cathedral, Martin Sheen, as President Jed Bartlet, delivers a blistering, fist-pumping diatribe, in Latin, to God. Viewers who weren't shaken by that probably can't be shaken ... or stirred.

-- When Toby learns that an old overcoat he'd given away had wound up on a homeless vet who died on a park bench, he arranges for the man to be buried at Arlington Cemetery. He sits by the coffin, alone, in a cold, blustery wind as the military guard plays "Taps" and the gun salutes echo.

-- At the end of one episode, the young staffers sip beer on the steps of a brownstone and marvel at the honour of working in the White House. One by one they repeat, with hushed reverence, their swearing-in pledge: "I serve at the pleasure of the President."