Joyce Blalock, a 56-year-old lawyer whose hobby is mountain climbing, immediately became nervous when her duties as D.C. inspector general led her to uncover evidence that could incriminate former District deputy mayor Ivanhoe Donaldson.

"But I didn't hesitate at all," she said yesterday. "Oh, I was nervous and I knew that we had to handle the case carefully. It was an enormously difficult case."

She eventually took her findings to Mayor Marion Barry, and the findings played a role in a year-long federal investigation that ended in Donaldson's guilty pleas Tuesday to charges that he stole about $190,000 from the city government.

Blalock, who left her District job in March to become inspector general for the U.S. Government Printing Office, said that the Donaldson case has become without question a highlight of her career.

"I feel that the system is vindicated," she said. "The mayor's insistence on an independent office of inspector general with a professional staff is a positive step toward good government . . . . Any system can have some people who succumb to some temptations, although in this case it was an extraordinarily powerful individual. I feel good about it."

It is only in recent days that Blalock has felt confortable about talking about her role in the Donaldson investigation. To those who worked with her in the District government, she was a no-nonsense investigator who demanded a lot of her 25-member staff and insisted that her office be viewed as "the focal point for integrity in the District government."

"She is a professional," said David Legge, the District's deputy inspector general, who worked with Blalock for five years. "She made an effort to achieve objectives of her job without being flashy or anything of that kind. We went by the regulations, and the Donaldson case was not a great deal different. We approached all of the cases with the same degree of thoroughness."

Yet the Donaldson case was different. Donaldson had been the mayor's most trusted adviser for a decade and had held top-level positions in the District government, including acting director of the Department of Employment Services.

"It is hard to say what a great shock it was," Blalock said yesterday of the Donaldson case. "It was not at all what we had expected that we would find. I knew instantly we had a serious matter."

Blalock had not been seeking information about Donaldson. She had begun an unrelated review of the financial controls in the Employment Services Department after the apparent abuse of the department's purchasing system by low-level employes.

During the review, Blalock and her staff began to examine checks from a special administrative account connected to the unemployment compensation fund.

"We were in my office and I knew instantly that we had a serious matter when someone brought me a file with Ivanhoe Donaldson's name on it," said Blalock.

During a telephone conversation, Blalock said, she notified Donaldson that she planned to interview him, would have two staff members present and would inform him that anything that he said could be used against him. Donaldson showed up in Blalock's office and insisted that he had no intentions of following those guidelines.

"It was tough," Blalock recalled. "He was evasive and difficult to deal with. He contested my authority to interview him and had to be convinced that I had the authority and the full backing of the mayor."

The interview was conducted according to the guidelines set by Blalock, who maintained that her legal training as a trial lawyer for the Food and Drug Administration and a former assistant attorney general for New Mexico had prepared her well to handle confrontational situations.

Blalock is divorced, has three sons and lives in Southwest Washington. Her future holds more audits and investigations of allegations of fraud, waste, abuse and mismanagement in the federal government -- work that she said is not unlike her mountain climbing hobby.

"You compete with yourself in mountain climbing," she said, "and there is a sense of achievement for those who do it that can't quite be shared with anyone else."