Former presidential adviser Thomas C. Reed was acquitted yesterday of criminal charges of fraud and obstruction of justice stemming from a $3,000 investment in his father's company that yielded a $431,000 profit within 48 hours.

After 10 hours of deliberations, a U.S. District Court jury in Manhattan rejected prosecutors' arguments that Reed used inside information from his father, a director of Amax Corp., in buying highly speculative Amax stock options in 1981, just before the company was targeted for a takeover attempt that sent its stock soaring.

The verdict followed a seven-day trial in which prosecutors portrayed Reed, 51, as a duplicitous man who carried on simultaneous love affairs with two women and arranged for them to share in his stock profits.

Reed's lawyer, Edward Shaw, maintained throughout the trial that Reed never received any inside information and that the charges were based "on the wildest speculation."

After the verdict was announced, Reed, wearing a rumpled, lightweight, gray suit, rose from the defendant's table and embraced girlfriend Kay Riddle, who was seated in the front row.

Reed was charged with one count each of securities fraud, wire fraud and obstruction of justice and faced a maximum of 15 years in prison. U.S. District Court Judge Robert J. Ward presided.

Albert Culler, one of the jurors, said afterward that the government had not proved beyond a reasonable doubt that Reed received insider information from his father. Culler said there was not enough evidence of what Reed and his father said to one another.

In 1981, Reed settled a civil suit with the Securities and Exchange Commission dealing with the same stock transaction. He agreed in a consent decree to repay the $431,000 in profits. Renewed allegations led to the criminal probe and his indictment last year.

Reed testified during the trial that he had received no inside information, but bought the stock options as a gamble on behalf of friends because he believed President Reagan's economic program would boost the stock market.

A California engineer, real estate developer and wine merchant, Reed is a longtime confidant of Reagan. He helped manage both of Reagan's campaigns for governor of California.

Reed was named an assistant secretary of defense by President Gerald R. Ford in 1974, became Air Force secretary the next year, and served as special assistant to Reagan for national security in 1982 and 1983.

Assistant U.S. Attorney Charles M. Carberry presented evidence that on March 4, 1981, the day he bought many of the Amax options, Reed spoke twice by telephone with his father, Gordon Reed, then 81. Carberry charged that Gordon Reed disclosed that he had been summoned to an Amax board meeting to consider a merger proposal, then the largest in history, from Standard Oil of California.

Three minutes after the second call, Reed phoned his broker in San Raphael, Calif., and bought 500 risky options requiring a sharp jump in Amax stock's price in less than three weeks to turn a profit. The takeover attempt pushed the stock price from $38 to $57 a share, and Reed sold his options March 6 for a $431,000 profit.

Gordon Reed was too ill to testify, but he denied in a deposition that he had given insider information to his son.

Reed gave the profits to eight people, including Riddle and another girlfriend he had at the time, as well as his wife's parents. He acknowledged in testimony that he had hired Riddle as his special assistant at the Pentagon and secretly lived with her while his wife was still in California. Reed recently separated from his wife.

As he left the courtroom, Reed said, "I'm delighted. This jury has confirmed what I have said since March of 1981. I have tried to be open and set forth the facts," the Associated Press reported.

Reed said he plans to return to his California business interests and does not want to rejoin the Reagan administration, the AP said.