John W. Hinckley Jr., who is charged with attempting to assassinate President Reagan, took an overdose of Tylenol yesterday in an apparent attempt to harm himself, according to a Justice Department spokesman.

Hinckley was examined, given an antidote to counter potential physical problems and placed under 24-hour watch at the federal correctional institute in Butner, N.C., where he has been undergoing psychiatric evaluation since April 2.

"There was some toxicity in his sytem but nothing to place his life in peril," said the spokesman, Tom DeCair. "He was at all times ambulatory and coherent."

Hinckley was rushed to a prison medical unit after telling a defense psychiatrist at 2:30 p.m. that he had taken the Tylenol, an aspirin substitute, which he had been secretly saving, according to DeCair.

It could not be determined how many tablets Hinckley swallowed, but tests showed that it was more than a normal dose, DeCair said. One government source described the amount as "a handful."

Taken in large amounts, Tylenol can damage the liver and cause death. Government sources said last night that Hinckley's action is being regarded as a serious attempt to hurt himself, rather than a ploy for attention.

Hinckley had been requesting aspirin for headaches almost from the time he was transferred to Butner, DeCair said. He also had been telling the prison staff recently that he was depressed, DeCair said.

Hinckley's parents had been at the office of U.S. Attorney Charles F. C. Ruff in the federal courthouse here yesterday for an hour when Ruff was notified about 3 p.m. by Norman A. Carlson, director of the federal Bureau of Prisons, that Hinckley had been taken to the prison medical unit.

Ruff privately relayed the information to Hinckley's attorney, Vincent J. Fuller, who was with the parents, and Fuller told the parents. All three left the courthouse, with the Hinckleys walking hand in hand, visibly shaken.

Sources said Hinckley's father was being interviewed about his son in connection with the investigation of the assassination attempt.

According to sources close to the prosecution, the younger Hinckley had become particularly despondent in the last few days after the Justice Department rejected an attempt by defense attorneys to arrange a guilty plea under the federal law dealing with young offenders.

Hinckley's attorneys had approached government prosecutors to discuss such a plea arrangement but were rebuffed last week, according to these sources.

The law provides an opportunity for offenders under 26 years of age to receive an indefinite prison sentence, providing for early release upon rehabilitation. Hinckley, who turns 26 tomorrow, faces life imprisonment if convicted without benefit of the youth offender provision.

Hinckley was sent to Butner for an estimated three months of examinations by psychiatrists appointed by government and defense attorneys.

Hinckley is charged with attempting to assassinate Reagan and assault on a federal employe, Timothy J. McCarthy, a Secret Service agent wounded in the attack outside the Washington Hilton hotel March 30.

D.C. police officer Thomas K. Delahanty and White House press secretary James S. Brady also were wounded. No charges have been filed yet in those instances.

Hinckley was initially placed under 24-hour watch at Butner, but that practice was discontinued shortly after his arrival there, DeCair said. He has been kept in isolation to protect him from possible harm from other prisoners, the spokesman said.

DeCair said the Bureau of Prisons is conducting an inquiry into how Hinckley could have saved and taken the medicine undetected.

DeCair said the incident is not expected to delay Hinckley's psychiatric testing.