NEW YORK (AP) - Hugh Downs has probably logged more time on the air than the test pattern, which, if you hadn't noticed, he long outlasted.

After tonight's ''20/20'' - the ABC News magazine Downs has anchored since its second week 21 years ago - this TV veteran will end a half-century run.

His last ''20/20'' (10 p.m. EDT) will be all his, including a final reported piece, a look back at his career and an interview by Barbara Walters, his co-anchor since 1984. Then Downs will be gone and conspicuous in his absence. Without him, TV will surely be a little less calm. For, above all, the constancy of calm has marked his reign no matter what, no matter when, whether as a morning-show host or a late-night sidekick.

The man ''The Guinness Book of World Records'' recognized for more on-air hours than any other TV personality (topping the 10,000 mark), Downs has been as much a fixture on your TV sets as the on-off button. After tonight he's off. Not retiring, mind you. Just moving to the next thing, a Web startup called iNEXTV that will cover the government's executive branch, he says, the way cable's C-SPAN covers Congress.

With its November launch, Downs will provide a weekly commentary for iNEXTV, thus helping to break in a new medium, just as he did in 1945 when he switched from radio to television to read headlines.

This, despite initial misgivings that ''TV was a gimmick, like 3-D movies, and would just go away.''

Downs is reminiscing for colleagues and friends at a dinner thrown for him last week at Manhattan's Museum of Television & Radio.

He recalls how as a youngster during the Depression he broke into radio in Lima, Ohio. He was paid just $12.50 per week and the program director leveled with him: He was terrible, at least at the beginning. But the gig sure beat the alternative, installing roofs.

He speaks of the kismet that delivered him to ''20/20.'' Its premiere on June 6, 1978, was an unmitigated disaster. The next morning, then ABC News President Roone Arledge scuttled its frenetic format, sacked its co-hosts and brought in Downs.

''It fell out of the sky, into my lap,'' Downs says gratefully.

On the screen behind him, a highlight film rolls:

Here is Downs interviewing a boyish Jerry Lewis on a 1955 edition of ''Home,'' NBC's innovative daytime magazine.

Here is Downs that same year anchoring what is perhaps NBC News' first-ever special on the space race: ''We know that the machines are going to be planned eventually that will take man himself into space,'' he reports.

Downs is seen in 1958 emceeing ''Concentration,'' which some 2,000 mornings he introduced as ''the television game where the ability to concentrate pays off.''

And on it goes: Downs riding shotgun as announcer for Jack Paar on his groundbreaking late-night talk show from 1957 through 1962; anchoring ''Today'' for nine years in the 1960s; serving as host for public television's ''Over Easy,'' a daily public-affairs show for seniors, in the late 1970s.

''I have been in the habit of repotting myself every decade,'' Downs explains.

And so he has again. He and Ruth, his wife of 55 years, will now relocate to their home near Phoenix, where he will begin logging hours in cyberspace.

But first ... praise and inquiries.

Andy Rooney, who began on CBS' ''60 Minutes'' a month after Downs bowed on ABC, rises to offer him a prickly bouquet: ''You're more lovable and interesting than I ever saw on '20/20.'''

''You're irreplaceable,'' says Walters, who should know: Starting next week, she will anchor Friday ''20/20'' solo.

And fellow ABC Newsman John Stossel confides that ''when I was first on with you, I was always astonished how calm you were - while I was scared out of my wits. Were you ever nervous on television?''

''I'm not at all nervous being on the air now,'' Downs says with his familiar smile, ''but that took a long time.''

For a reporter a few days later, he divulges how.

''When I was a kid in Ohio, I got snake-bit,'' Downs says. ''I didn't know whether or not it was a water moccasin, and heading home to find a doctor I was panicking and tempted to run.'' This would have hastened the spread of poison, if any, through his system. ''I forced myself to walk.''

He soon learned the snake wasn't poisonous. And he also learned a lesson: ''If you pretend not to get upset, you WON'T get upset.''

It seems the lesson never failed him or his 50 years of viewers.

EDITOR'S NOTE - Frazier Moore can be reached at fmoore ''at'' ap.org