Experts on treating alcohol abuse have these words of advice about Defense Secretary-designate John Tower's pledge of abstinence: Believe it when you see it.

Marc Hertzman, professor of psychiatry and behavioral sciences at George Washington University, said it is easy and common for heavy drinkers to take the pledge but rare for such vows to be kept."Let's assume that only a very small part of what has been rumored about the gentleman has any validity to it at all," said Hertzman. "The notion that he could just stop is about 180 degrees counter to anything that anybody knows about the problem of dealing with drinking alcohol."

Tower, whose nomination to become secretary of defense comes before the Senate this week, has admitted to having been a heavy drinker in the past. But in an extraordinary television interview on Sunday, he vowed not to drink "beverage alcohol of any type or form, including wine, beer or spirits of any kind" if he is confirmed to the Cabinet position.

"I'm sure he intends every word that he says. The question is, can he do it?" said Hertzman. "I think the Senate would be ill-advised to draw conclusions from that vow."

He said that in some cases, six months of total abstinence would be convincing. With others, he said, it could take two years or more, or even a struggle that lasts a lifetime.

"If I were a senator, I think at the very least I would defer his confirmation until he proves that he can go six months," said the psychiatrist. "That's a minimum figure to establish that you don't need it. A year and a half is more like it."

Dr. William E. Flynn, director of the alcohol and drug abuse clinic at Georgetown University, said that Tower's statement simply is not persuasive to professionals in the alcohol treatment field.

He said if senators have to base their confirmation decision on Tower's drinking history, "it would be a tough one."

"What a corporation would do (for an executive in a similar situation) is decide that there must be an evaluation. But that's not possible here," said Flynn. "One would feel a lot more comfortable if you get an evaluation and a clean bill of health."