Maryland Gov. Marvin Mandel today proposed that inmates confined to the controversial Patuxent Institution for psychiatric behaviour modification be allowed to transfer to other penal institutions whenever they wish.

The proposal, presented as an amendment to a Mandel bill that would have required inmates in the institution to serve out their sentences there, caught the bill's opponents - who had come to testify against it - by surprise.

That amended bill, if passed, mark in effect the end of a 22-year-old experiment that saw persons adjudged to be "defective delinquents" and habitually violent street criminals incarcarated in the institution for as long as Patuxent staff members felt was necessary, no matter how long the inmates' criminal sentences were.

The experimental approach, in which psychiatrists attempted to modify the violent and antisocial behavior of the inmates, won the unique program widespread criticism from civil libertarians who critized the open-ended sentences.

Mandel's original proposal to modify Patuxent would eliminate the term "defective delinquent" in favor of the term "eligible person" to designate inmates at Patuxent, and provide that any Patuxent inmate be released after serving the criminal sentence given him.

Under the amendments proposed by Mandel legislative aide Alan Wilner today, an inmate who wished to leave Patuxent simply would request a transfer in writing, and be moved to another prison in the correction system.

Under Mandel's bill, said Wilner, "Patuxent is simply an alternate place to serve. The sole consideration in getting in to Patuxent is how best to rehabilitate a person, not to extend his period of incarceration . . ."

Wilner predicted the bill would result in "a complete turnaround and those groups that have been suing for 25 years to get people out of Patuxent will start suing to get them in."

A prisoner attempting to win admission to Patuxent would have to have a sentence with at least three years remaining on it, an "intellectual deficiency or emotional imbalance," and be adjudged likely to respond favorably to Patuxent's analytical and behavior modification treatment capabilities. Prisoners could also be ordered sent there, but they would have the right to a transfer if they wished.

Mandel's bill was based on the conclusions of a seven-month, $120,000 study of the institution mandated by a law passed last year. The study was conducted by Contract Research Corp. of Belmont, Mass.

The study concluded that the indeterminate sentence "has been shown to result in offenders' being incarcerated for longer periods, owing to an inability, thus far, to predict dangerousness accurately and the consequent tendency to over predict." The study found "no valid or reliable basis for the indeterminate sentence."

"The indeterminate sentence and the concept of defective deliquency should be repealed, and the present program at Patuxent Institution should be significantly modified," the study reads.

Committee vice chairman Melvin A. Steinberg (D-Baltimore County) said the committee probably will approve Mandel's bill and the amendments.