"When I became a cop," recalls Montgomery County Police Chief Bernard D. Crooke, "the first two words I learned were, 'Yes, sir.'"

"Nowadays," he says, "the first words you learn are 'Why?' and 'Why not?'"

Crooke tells that story in an effort to explain how being a police chief has changed over the past two decades from a cushy, semiretirement post into a demanding, virtually impossible job where few seem to last.

Four police chiefs in the Washington area have left their jobs, one way or another, in the past 18 months. None was over 50 years of age, and none had been on the job more than four years.

District of Columbia Police Chief Maurice Cullinane complained of a bad knee, Prince George's County Police Chief John W. Rhoads cited a bad back, Arlington Chief Roy C. McLaren decided to go to London to study and Montgomery Chief Robert J. diGrazia was fired.

Whatever the stated reasons, there was a common underlying theme to their departures. Eventually, the demands of the job simply wore them down.

"This is a burnout profession and chiefing has been a burnout job," Crooke says, "I hope I last five years, but who knows?"

Where police chiefs often used to hold their jobs 10 or 20 years, statistics show the average police chief today lasts only 2 1/2 years.

Where police chiefs once held the ultimate authority over their departments, chiefs today are facing - and losing - increasingly frequent votes of confidence among the rand-and-file.

And in the past decade, police forces - and their chiefs - have been pushed increasingly to the forefront of the often volatile world of local politics.

The job thus has become anything but a comfortable retirement post, the chiefs say.

"You have to decide right off the bat that you are not going to be able to work some kind of five-day, 40-hour week," Rhoads says. "Any chief who tries to do it that way is crazy . . ."

"I can remember one night I had come home late. We had been working on the budget. I was exhausted and went straight to bed. I couldn't sleep. I was nervous and sweaty. Finally, about 2 a.m. I got up, got dressed and went downstairs.

I just sat there for a while, staring at the phone. I knew it was going to ring.Just as I got up to get in my cruiser to drive towards work, it rang. I picked it up and without hearing a word I knew something awful had happened. One of my men had been shot.

"I still think about that night a lot."

The biggest problem Rhoads faced in Prince George's was winning rank-and-file support for major changes in procedure. This task of selling new ideas to police officers has been a significant addition to the job of many chiefs: departmental changes were the chief cause of the "no confidence," votes faced by Rhoads and diGrazia last year.

Around the country, 11 other chiefs faced similar "no confidence" votes in 1978.

But the rise in the strength of police unions is just one trend of the late 1970s that has changed the job of chief. A number of other changes in police work have made chiefing the "burnout," job it now is. They include:

The civil rights movement. For the first time police around the country were given nationwide exposure. Television news audiences saw police officers clubbing demonstrators, night after night. Now, police say that these highly publicized incidents were the result of undermanned, undertrained departments trying to cope with unfamiliar problems.

The antiwar movement. For police, this was a sequel to the civil rights movement, only worse. "It wasn't quite that bad when people saw blacks in Alabama or Mississippi being thrown in jail," said David Couper, Chief in Madison, Wis. "But when they turned on their TVs and saw their children being thrown in jail they started saying, 'My God, who are these people?'"

Affirmative action. Up until the late 1960s most police departments were lily-white and almost totally male. These policemen were unhappy to find blacks and women joining them.

An influx of well-educated officers. Because of the social changes and the newly skeptical attitude toward police, local governments began taking steps to hire officers with college degrees. But well-educated officers were more inclined to question the orders they were given.

A more aggressive press corps. In the wake of the civil rights and antiwar movements, the press became highly skeptical of the police. ". . . The chief was all of a sudden the spokesman for the department," Crooke said. "If he said something was so, it better be right because if not, he was going to have to answer to the press."

The rise of police unions. This is a recent phenomenon which may only be starting to peak. Union leaders have often emerged as outspoken opponents of the chief, undermining his authority with the street officers.

Of the four local chiefs who retired recently, only one, Cullinane, retained broad popularity among his men and local politicians as well. Cullinane points out that he and the chiefs who preceded him in Washington had a major advantage - the tremendous influx of new men in the department in the late 1960s and early 1970s.

The District's massive hiring plan was inspired by former President Nixon's "crime capital of the world" speech of 1969. Most other urban forces have faced tight budget constraints that have left them unable to increase their manpower.

"When you take one or two men in each year, it's easy for the old hands to grab them and mold them into the kind of policemen they think they should be," Cullinane said. "The old hands dominate.

"But if you have an infusion of new men in huge doses like we did in Washington, they figure out their own ways."

To make changes, police chiefs must try to accommodate not only their own men but their political bosses and the public they serve as well. And all of these constituencies are more sophisticated than they used to be.

"The thing you have to remember is that being a police chief has always been a political job," said Richard D. Hongisto, Cleveland's former chief who resigned late last week as New York State commissioner of corrections. "Only with all the changes in policing in the '70s and the new complexities of the job, it's even more political than ever before."

Honigsto, for example, was Cleveland's eighth chief in nine years. Considered a maverick in police circles, he quickly became embroiled in a battle with Mayor Dennis Kucinich. After less than a year, a ninth chief in nine years succedded him.

Hongisto's experience is now almost average. But before it had nine chiefs in nine years, Cleveland had one chief in 10 years.

Montgomery County has now had four chiefs in three years. Before that, one chief was on the job 14 years. In Prince George's there have been five chiefs in 10 years. For 15 years prior to that, one man held the job almost continuously.

The man who many think of as the prototype of the chief of the '70s is Patrick V. Murphy. Murphy, now president of the Police Foundation, headed departments in Syracuse, New York and Detroit and was director of public safety in Washington, D.C. His average stay in each job was 1 1/2 years.

Murphy says today that while it would be nice for a chief to be beloved by his men it is often impossible.

"I've always said that this isn't a popularity contest," Murphy said "It's nice to have input from the men and you try and make them understand why you are doing something.

"But sometimes they make it clear they don't want some change . . . You have to go ahead with it anyway because you think it's right."

One widely held view is that chiefs have trouble making certain - cutting down on manpower, changing the rules governing the use of force and firearms, or breaking up patrol teams - because most policemen are resistant to change. "They're taught to defend the status quo," Murphy said. "They get used to doing that, on all levels."

Among the area chiefs, it was Cullinane who was best able to sell his changes. He was popular with the men, with the press and with most of the community, an increasingly rate combination.

On the other end of the spectrum is diGrazia. Flamboyant and quotable, diGrazia, now 51, rode into Montgomery County on a wave of press clippings, many of which mentioned him as a potential candidate for the FBI director's job.

He had brought changes to departments in St. Louis County and Boston and left those areas popular with the citizenry but unpopular with the men. Quickly, the pattern repeated itself in Montgomery.

"This should have been the ideal place for Bob," a friend said recently. "And it would have been if he had been able to marshal the forces in the community which would have been inclined to support him. But he never did that for some reason. When the rank and file went after him, no one rose to support him."

"Everything he tried to do had been done somewhere before with success," one chief said. "But diGrazia wasn't just trying to reform the Montgomery County police department, he was trying to make a national name for himself. So he said a lot of things that were going to be quoted around the county and were going to get the men angry at him."

DiGrazia himself has a lawsuit pending against the county and so refuses to discuss his tenure as chief.

Part of a chief's job now is creating a positive image for his force. In Prince George's County, "there is no way the situation with our department could have been as bad as our image was when I took over," said Rhoads. "Nothing could be that bad."

"But one of my jobs was to change the department's image. Sure, that was a public relation job. I probably didn't do it very well either. Because now, our image should be a lot better than it is, based on the work we're doing."

But even if a chief can somehow keep the politicians, the officers, the public and the press happy, Cullinane believes he shouldn't stay very long anyway.

"No chief should stay more than four or five years," he said. "Because once you've been there that long you've shaped the department in your own image, it is a reflection of your idea. Once that happens it's impossible for you to recognize the problems that are there and see the flaws because you probably created those problems or flaws . . .

"After four years most guys want out anyway. You're in a very exposed position. You always have to be covering your flanks on all sides. It's exhausting." CAPTION: Picture 1, JOHN W. RHOADS . . . resigned in Prince George's; Picture 2, ROBERT J. DiGRAZIA . . . fired as Montgomery's chief; Picture 3, BERNARD D. CROOKE . . . Montgomery police chief; Picture 4, ROY C. McLAREN . . . left Arlington for overseas study; Picture 5, MAURICE CULLINANE . . . retired as District chief