LOS ANGELES, Feb. 20 – An associate of Charles Manson was arrested in a courtroom on suspicion of being under the influence of drugs Thursday when he showed up in what prosecutors suspected was a narcotic-induced stupor. He was not booked after undergoing police tests.

Assistant district attorneys summoned narcotics officers to Superior Judge D. Sterry Fagan’s courtroom in Van Nuys when Steven Grogan, 18, was unable to respond to the jurist’s questions.

Grogan stood motionless, staring into space, as Judge Fagan twice asked him if a proposed substitution of attorneys was “agreeable” to him.

Judge Fagan was prepared to rule on a motion that Daye Shinn, an attorney who has frequently visited Manson in County Jail, replace Grogan’s previous attorney.

The judge also planned to set a date for Grogan to be tried on an auto theft charge.

But both matters were left up in the air after Grogan’s arrest Thursday, and must be dealt with at some future date.

After Grogan appeared unable to understand Judge Fagan’s questions, Shinn hustled him out into the corridor where they huddled with two female and one male member of Manson’s Family.

As Shinn talked to him, Grogan nodded his head slowly and mumbled to the attorney:

“I’ll say yes.”

But when he returned to the courtroom, Sgt. John Collella and Officer Roger Dunn led him back out into the corridor, followed by Shinn, who protested that Grogan was dazed simply because of a lack of food and lengthy jail confinement.

The officers arrested him, however, after subjecting him to brief preliminary tests and finding his eyes “unusually dilated.”

But further tests failed to establish conclusively that Grogan was “under the influence” and he was not booked.

According to Susan Atkins, principal witness against Manson and five co-defendants charged with the Tate-LaBianca murders, Grogan accompanied the accused to the LaBianca home last Aug. 10. However, Miss Atkins said he did not enter the home.

Grogan, also known as Garth Tufts, Grant Mollon and “Clem,” was arrested last fall during a raid on the Death Valley ranch, the headquarters of the Family at that time.

By JERRY COHEN