The Glen Burnie woman charged with lying to a federal grand jury here testified today that her alleged attempt to buy secret information about the investigation of former governor Marvin Mandel was a joke.

In 3 1/2 hours of testimony in U.S. District court, Donna B. Brown, 35, insisted she was trying to tease her best friend, Diane Lawrence, who was one of the grand jurors investigating Mandel in 1975.

"There wasn't any reason to tell Diane it was a hoax," Brown told prosecutors on cross-examination. "She knew it was . . . she didn't question it for three long years."

Brown's testimony contradicted that of Lawrence, who testified yesterday and today that she did not view their 1975 conversations as one of Brown's many "pranks."

Lawrence said she allowed the FBI to tape her conversations with Brown in case someone actually did want to buy information from the grand jury. After three weeks, when no one offered the $5,000 "up front" money she and Brown had discussed, Lawrence said she assumed "they didn't want to got through with it."

One of Mandel's codefendants, Harry W. Rodgers, said through his attorney last week that he was "approached" with an offer to buy the information, but told the man he "wasn't interested."

Both Brown and Lawrence said they continued a "close friendship" until last summer.

Brown said she and Lawrence had "many other goofy conversations" and, even though her "joke" on Lawrence continued for three weeks, she did not remember it when she was called before the 1978 grand jury investigating the alleged tampering with the Mandel case.

"The entire time I was in front of the grand jury, I had the feeling everybody knew something but me," Brown said. "But of course everybody heard the tapes-everybody but me.

"Perhaps I was guilty of bad judgment. But I didn't bribe anybody."