D.C. mayoral aide Joseph P. Yeldell testified yesterday that he had converted part of a secret loan from millionaire Dominic F. Antonelli Jr. into about $17,000 in $100 bill, locked the money in a wall safe in his home and failed to list it in a financial statement submitted to the Small Business Administration.

In his third day of testimony at his federal bribery and conspiracy trial, Yeldell, 46, the city's former human resources director, told the Jury that he had initially intended to save the money to pay for his older daughter's college education, but he added that he later spent it on himself, mainly to pay legal costs for his trial.

"The money is all gone now," Yeldell said in a somber tone near the conclusion of more than 4 1/2 hours of cross-examination by the prosecution yesterday.

Yeldell's failure to report the money he had recently obtained as a result of Antonelli's loan in his July 1976 financial statement to be SBA appeared significant because Yeldell was arguing at the time that he could not repay the federal agency for a loan on which he had defaulted. At the time, according to court testimony, Yeldell owed SBA about $75,000.

In his testimony yesterday, Yeldell said that he did not believe he was required to report the money to SBA because he had set it aside for his daughter, Gayle. "It wasn't hidden from SBA. It was no longer mine," he said. "I gave it to my daughter."

Under increasingly pointed questioning by Assistant U.S. Attorney Richard L. Beizer, Yeldell frequently contradicted or disputed testimony by city government officials and other witnesses. Occasionally, he contradicted his own U.S. District Court testimony.

Yeldell also acknowledged further inaccuracies in financial statements he had prepared for banks and other institutions, and he admitted additional errors in statements he has made during a tape-recorded interview last year with city auditors who were investigating his dealings with Antonelli.Yeldell, who is now on unpaid leave from his job as a top aide to Mayor Walter E. Washington, had conceded similar misstatements on Wednesday.

Yesterday's cross-examination of Yeldell appeared sharper and sometimes more heated than it had on Wednesday. Several times, Judge Gerhard A. Gesell reprimanded Assistant U.S. Attorney Beizer for asking "argumentative" questions and rebuked Yeldell for giving "unresponsive" answers. At one point, as Beizer started to ask a question, Yeldell shot back, "Mr. Beizer, you interrupted me in a statement."

At the end of the court session, Yeldell told reporters he was tired. He had been on the witness stand for more than 12 hours since Tuesday. Yeldell's lawyer, John A. Shorter Jr., Left uncertain yesterday whether he would recall Yeldell to the witness stand this morning in an attempt to rebut prosecution charges.

In its cross-examination, the prosecution seemed to be stressing apparent inconsistencies in Yeldell's testimony in an attempt to damage his credibility in the jurors' minds. It also appeared to be seeking to point up evidence that Yeldell had concealed his relations with Antonelli from government and other officials - a central point in the charges on which Yeldell and Antonelli, a real estate developer and parking company owner, have been indicted.

The two men are charged with conspiring corruptly to secure a lucrative. 20-year city government lease of a building at 60 Florida Ave. NE from a partnership controlled by Antonelli. The building was leased in 1976 for the D.C. Department of Human Resources, which Yeldell headed at the time. In return for Yeldell's aid in arranging the lease, Antonelli is accused of secretly giving the former DHR chief a $33,000 loan after helping him obtain a series of previous short-term bank loans.

It was part of Antonelli's secret $33,000 loan that, according to Yeldell's testimony yesterday, ended up in a wall safe in Yeldell's home in 1976 after what has been described by other witnesses as a series of intricate bank transactions. Yeldell had previously testified that he did not know at the time that the money had been provided by Antonelli. In public records, the loan was listed in the name of a "straw," a fictitious person.

Although Yeldell's DHR chauffeur and errand runner, Joseph B. Turner, testified last week that he cashed two checks at a bank and then returned to yeldell's office with more than $21,000 in cash, Yeldell has never previously said publicly what he did with the money.

Yeldell acknowledged under cross-examination that despite his plan to set aside the money for his daughter's education, he had not told his daughter about the gift and had later spent it on himself. "I didn't tell her - she was 16 years old," he said at one point. In explaining why the money was spent for legal costs, Yeldell noted, "This case has been very expensive."

Part of the prosecution's questions yesterday appeared to raise further doubt about Yeldell's account of how the $33,000 loan was arranged. Yeldell, as he acknowledged yesterday, has previously contradicted testimony by Lawrence A. Sinclitico, a title insurance company employe who handled the loan transaction.

Yesterday, Yeldell initially said that loan documents had been notarized at his DHR office, but he later acknowledged that they had, instead, been notarized at the home of his private secretary, Lillian Manson.

The prosecution produced two loan settlement sheets yesterday, showing conflicting financial information. The prosecution also showed that a date had been altered on one document. Yeldell acknowledged that he had signed his wife's name on loan records.