Dr. Vincent Montemarano was acquitted yesterday of a charge that he injected a lethal dose of potassium chloride into a 59-year-old cancer patient at Nassau County Medical Center in New York, December 1972.

The controversial "mercy-killing" trial came to an end after one hour of deliberation by the jury, which determined that the state did not present enough evidence to prove that Eugene Bauer, a Long Island Rail Road engineer, was alive prior to the lethal injection.

Nassau County Judge Raymond Wilkes had earlier made the decision that, in order for a murder conviction to be sustained, District Attorney William Cahn would have to show that Bauer had not died of cancer before the dosage. Cahn had repeatedly termed the Montemarano act a mercy killing and had later described it as "an act of convenience."

J. Russell Clune, the defense attorney, insisted that Bauer had died of natural causes. The trial, the first such "mercy killing" action involving a doctor, lasted 12 days.