The National Security Agency, reputed to be the country's most secret intelligence-gathering organization, put a "Help Wanted" sign in the window this weekend, and hundreds of aspiring sleuths showed up to apply for jobs.

Never before had NSA held a public "career fair" to woo scientists into the profession of international snooping, but "expansion into new areas of high technical complexity" has prompted the agency to seek experienced specialists more aggresively, said Addie Paul, the NSA spokesman at Saturday's daylong reception in the ballroom of the Columbia Inn in Columbia, Md.

Just what areas NSA is expanding into, Paul would not say.

Smiling and shaking his head, he also declined to reveal how many recruits he was looking for, what they would do if hired, or what qualifies an engineer or computer scientist to work for NSA.

"We don't go so far as to deny that we exist," said Paul, surveying the fair's colorful information tables, impressive computer displays and long lines of job applicants. "But usually we like to work quietly.

"Today," he added, "is a big exception."

There were no trenchcoats and no secret-code rings. Instead, applicants juggled neatly typed re'sume's and complimentary cups of coffee.

Attracted by newspaper and radio enticements to "meet the challenge at NSA," they ranged from recent college graduates hungry for any kind of well-paying employment to middle-aged PhDs bored with their current jobs. About one-fifth of the applicants were women.

Some of the applicants seemed more nervous about NSA's going public than the agency officials themselves, who could be identified in the large crowd by their convention-style orange name tags.

"I can't talk to you. This is secret," said one man waiting in line at the "Tactical Systems" booth. Then he proceeded to explain that after 22 years as an electrical engineer in Baltimore, he wanted "to get into something new, probably like most of the people here."

Beth Zimet, 38, a computer technician at the Goddard Space Flight Center in Greenbelt, was less intimidated by NSA's reputation.

"Some things have to be investigated because they're just not kosher as far as this country's interests are concerned," said Zimet. "If some terrorist is going to leave a bomb somewhere, I'd like our security to know first, wouldn't you? And I would rather have me as a spy, an honest, patriotic, upstanding person, rather than some slimy character."

"The main thing is to carry on the national defense as best we can," said a 16-year Air Force veteran who declined to give his name "because that wouldn't be right, me here looking for a new job."

"This is a good group; they know their stuff," said another man on his way out. He added only that he now works "for the other guys--you know, the CIA."

Paul explained that because the government generally pays scientists less than they can make at major corporations, NSA has to offer "something different, something exciting." He added, however, that those who have notions of James Bond-style adventure "may be a little disappointed. A lot of it is just plain computer work."

NSA's reputation for offering the tops in microchips was precisely the reason many people had come.

"They have all the good stuff--state of the art in computers," said Craig Kelly, 21, a recent graduate of Towson State University in Maryland.

Explaining his own goals, Kelly said, "Research would be fun, code-breaking I guess they do a lot of, but I'm not into spying on other countries. Not for me."

Don Joy, the NSA employe standing at the Advanced Microwave Receiving System booth, said that in the agency "you can take what you learn in school or in civilian job experience and do something most people wouldn't even think about."

Asked about his own field, Joy said, "I'm in operations," paused, then added, "That's enough; that's already more than I should say."

Despite his concern over inadvertently leaking sensitive information, Joy didn't seem worried about the possibility of nosy foreign rivals at the fair.

"We're taking a chance, sure," he said. "But we've got to meet our job requirements."

Ray Cook, an NSA expert in microelectronic fabrication processes, described his work as "building the insides of very little things--the size of your watch, for instance."

While a small group watched, he narrated the construction of a minuscule circuit, from the drawing of a logic diagram to the completion of the "nomenclatured part."

But NSA doesn't make watches, he conceded. "That's just a comparison I use, and that's where I stop."

After a series of rigorous interviews and a three- to six-month security check, a few of the applicants may someday have careers in foreign intelligence, communications security or computer security--none of which they'll be allowed to talk about, said NSA's Paul.

Although most NSA employes are based at Fort Meade in Maryland, he emphasized that the agency's "mission is foreign. We don't do our work at home, no matter what some people may think."

Edward O'Keefe, a civilian electrical engineer for the Navy, said he came to the jobs fair to "keep my options open, so that if I have to change jobs it's my choice."

Unimpressed by the allure of covert intelligence work, he said, "Look, I'm in computer hardware. There are no real mysteries involved here in hardware."

He smiled, adding, "Well, there are some mysteries, but there are mysteries where I work, too."