A jury of five men and seven women was chosen late yesterday to hear the case against John W. Hinckley Jr., the man accused of attempting to assassinate President Reagan. Within an hour after the process was complete, the prosecution announced that Reagan will not be a witness at Hinckley's trial.

U.S. Attorney Stanley S. Harris said in a statement that his office had acquiesced in a decision made at the White House that Reagan would not appear in the case, which is expected to begin today with the presentation of the government's evidence.

Harris said Hinckley's defense lawyers had consistently opposed any testimony by Reagan, who was wounded in the chest during the attack more than a year ago. While Reagan remains willing to videotape his testimony, Harris said his office decided against that option since it would result in legal controversy that would further interfere with Hinckley's long-delayed trial.

Harris said in the statement that the White House decision was made after consideration of the demands posed on Reagan's time by "domestic and international problems of great inportance," concern about Reagan's security and the knowledge that Reagan's testimony was not essential to the fundamental issues in the Hinckley case.

As a result, Harris said, "It is not now expected that he will be an active participant in the trial."

Harris said that he and the chief government prosecutor, Assistant U.S. Attorney Roger M. Adelman, had met with Reagan last March and discussed the case at "considerable length" with him. Harris said that while the jurors will not hear testimony from Reagan directly, they will receive evidence about Reagan and "the effect of the events upon him" from other witnesses.

Hinckley contends that he was legally insane when he shot and wounded Reagan, his press secretary James Brady, D.C. Police officer Thomas K. Delahanty and U.S. Secret Service Agent Timothy McCarthy.

The selection of a jury to decide Hinckley's case took five days, much of which was taken up with private questioning of all the prosective jurors about their knowledge of the case and their personal experiences with mental illness.

One of the jurors selected for the case studied psychology as a graduate student at the University of Wisconsin and has taken courses dealing with abnormal psychological problems of socially maladjusted persons.

During limited questioning in open court, the woman told U.S. District Judge Barrington D. Parker that she works as a research assistant at the Armed Forces Institute of Pathology and has previous work experience at facilities for disturbed adolescents in Wisconsin and in the Washington area.

The jurors were called by number during the lengthy selection process and their names have been kept secret by court officials. Limited background information released by the court showed that the 12 jurors chosen range in age from 22 to 64 years old. Eleven of the jurors are black and one female juror is white.

Four of the 12 jurors work for the government, six are employed in private industry and two are retired, officials said. In addition to the research assistant, the jury includes two secretaries, a mechanic, a garage attendant, a banquet worker, a food service technician, a custodian, a supply specialist, and an information control specialist. One of the two retired jurors worked as an industrial specialist, but the second retired juror's occupation was not disclosed by the court.

Six alternate jurors, five women and one man, also were selected yesterday, in case one of the 12 jurors chosen has to leave the case for some reason during the trial. According to the court, those alternate jurors, all of whom are black, include a postal clerk, a docket clerk in the local court, a secretary, a retired budget analyst and a bookkeeper. One alternate juror's occupation was listed by the court as "unknown." Hinckley was present yesterday during the jury selection process, straining at times to get a look at the jurors who were being chosen to decide his case. As he was led from the courtroom, his hands shoved down into the pockets of his grey trousers, Hinckley glanced up only briefly at the 12 jurors finally selected.

Meanwhile yesterday, the prosecution asked Parker to block the defense from presenting testimony from a doctor who treated Hinckley after Hinckley attempted suicide last November while in custody at the army stockade at Fort Meade.

In court papers, the prosecution contended that such testimony would prejudice the jury in Hinckley's favor because it would suggest a relationship between his mental condition during the assassination attempt and his state of mind when he attempted suicide seven months later.