Six weeks ago, actor Stacy Keach was serving time in Reading Gaol. Yesterday he spoke calmly about how his abuse of cocaine started with a casual introduction to the drug at a social gathering and escalated into a nine-year habit that was broken only when he was arrested in Heathrow Airport, outside London, for possession of 1.3 ounces of cocaine.

"It wasn't until I was apprehended . . . that the shock of recognition finally jolted me into realizing that the drug was controlling me, and that I was helplessly at its mercy," Keach told the House Select Committee on Narcotics Abuse and Control at a hearing yesterday morning on cocaine abuse.

The actor, who most recently portrayed Mickey Spillane's fictional, hard-boiled detective Mike Hammer in CBS' highly stylized, slightly hammy series, said he could "only thank God that I now have an opportunity to speak out to others -- in the hope that they will not have to travel down the same road as I did."

In his most extensive public appearance since his release June 7 from Reading Gaol, Keach also answered questions at a 20-minute press conference in the Rayburn Building hearing room.

Asked how his on-screen character would react to the actor's cocaine addiction, he said, "We're both one and the same person at the moment. It's a terrible problem that requires a lot of personal self-discipline to overcome . . . Mike Hammer is a very vulnerable human being. He makes mistakes."

Looking fit and wearing a light gray double-breasted suit, Keach, 44, said he would be doing a two-hour CBS television movie, "Mickey Spillane's Mike Hammer: More Than Murder." CBS has not said whether it will renew the series, which it has suspended.

Keach also said he plans to do a documentary about cocaine as well as public service announcements. "My greatest rehabilitation is sharing myself . . ." he said. "As long as I share myself, I feel safe."

During the hearing, Keach cited Nancy Reagan's "exemplary" work in raising public awareness of drug problems, and during his press conference said the president's wife had called him. "She said she was sorry she couldn't be here to hear the testimony. I certainly understand why."

Despite his abstinence from cocaine, Keach said the temptation is still present: "This is not to say you're free of cravings for it . . . You have to monitor yourself . . . It's a day-by-day situation."

He described his cravings this way: "It may be just a flash. I don't know if you've ever smoked cigarettes. It's like, 'Gee, I'd like that.' You use metaphors to deal with it. I've come to equate that white powder with death. When I have that flash, I associate it with that. It's a day-by-day process."

His return to California and the entertainment industry will be "a real test of my self-discipline," Keach said. "I'm going back to the same environment. That's going to be a real test for me. I hope I can walk as I talk."

Keach, who was educated at Berkeley and the Yale School of Drama, made his Broadway debut in 1969 in Arthur Kopit's "Indians," in a role he created at Arena Stage. He has won three Obies for his off-Broadway work and starred in the national tour of "Barnum," which required him to walk a 35-foot, half-inch tightrope six feet off the ground each night. His re'sume' lists 19 movies.

He was accompanied on his visit to the Hill by his girlfriend, Malgosia Tomassi, a blond and deeply tanned Polish-born actress; Sidney Feinberg, who identified himself as Keach's friend and attorney; and a publicist, who shooed away reporters.

With a rueful smile, the actor described his first encounter with cocaine as a dalliance. "I thought, 'This is amusing, harmless.' " A year later he had his second encounter, he testified, and then "within a few short months, cocaine became an integral part of my life."

Keach said that he never believed he would lose control over his use of it: "I felt that I had a shield of armor. That's the terrible thing about cocaine. It almost encourages you to lie to yourself. Even when I started taking it -- in the mid-'70s -- it was considered not very dangerous."

He began using the drug to revive his energy, he said. "It was basically something to alleviate exhaustion and depression. The cocaine would create a false sense of energy. Then there would be a terrible depression, and you would take the cocaine to alleviate the depression . . . It became a vicious cycle."

He estimates that his cocaine habit cost him "$50 a day to $250 a day. It depended.

"I wasn't a heavy user," he said. "I was a continuous user . . . Sometimes I would use a quarter of a gram. In the early stages of my addiction, it would be half a gram or almost a gram a day. The main thing was that I used it regularly with very few recesses."

At the hearing, two other former cocaine abusers -- former Minnesota Vikings football player Carl Eller and Washington, D.C., bank employe Bernice Carrington -- echoed Keach's words about the power that cocaine once held over their lives.

"At one point, I was one of the highest paid defensive linemen in the NFL," said Eller, who now works as a consultant to the National Football League on athletes' drug problems. "I was making about $100,000 a year. Almost my total income was going to cocaine. "

Carrington, who said she had spent as much as $2,000 a day on drugs, told harrowing tales: "I was placed in an institution, I have been in jail, I've been in situations I can't tell you about."

Keach spent six months in Reading Gaol, from Dec. 7, 1984, to June 7, 1985. "I was very well treated," he said. "I was not attacked," he added, denying a widely circulated report.

Keach said he received help for his drug problem. "I did seek medical help. There's an amino acid . . . Apparently, it's had some positive results. I took it for about two weeks." (He mentioned a substance that affects the synthesis of serotonin, a brain chemical conveying messages from one cell to another.)

"I didn't go through a traditional terrible withdrawal period," Keach said. "I think that varies from individual to individual. That's not to say I'm in any way undervaluing cocaine usage. I still had terrible problems getting off drugs."

Keach credits his Bible study classes in prison with giving him "sustenance" and said he has become more religious. "I was always a Christian . . . ," he said, "but whenever you experience that kind of shock and trauma, it brings you to your knees."

He also got help in prison from a program called Cocaine Anonymous: "It's a matter of sharing experiences. You sit and share testimony. You let the guilt out, the feelings out. It's a support mechanism."

Keach said friends have told him that his drug problems and imprisonment have influenced them: "Friends of mine that I've known were involved with drugs have said, 'Stace, you did a lot of time for a lot of people.' "