More than half the convicted murderers released from state prisons in 1983 had served fewer than seven years behind bars, according to a government study released yesterday.

Half the rapists released had served fewer than four years in prison, and the median time served by all offenders in state prisons was 19 months, according to the study of prison terms in 30 states.

The survey of more than 300,000 criminal cases, released by the Bureau of Justice Statistics, also found that 18 percent of those sentenced to a term of life imprisonment served three years or less before being released.

The median time served on a life sentence is eight years and seven months, said the report by the bureau, a research arm of the Justice Department.

Bureau Director Steven R. Schlesinger said the 1983 figures showed no change from the previous year but were the culmination of a 56-year trend toward more lenient sentences. From 1926 to 1982, he said, the median time served for all offenses dropped from 19 to 16 months. Schlesinger said that prison overcrowding may have contributed to this trend.

Deputy Assistant Attorney General James I.K. Knapp cautioned against reading too much into the findings because of differences between state prison systems and definitions. For example, the "murder" category includes vehicular homicide.

Knapp said he was "disappointed" in some of the median sentences, particularly for rape, which he called "much lower than it should be." But he said there is a national trend toward stiffer sentences.

Prisoners with life sentences who were released in 1983 served more time in Maryland -- an average of more than 12 years -- than in the 19 other states reporting. The shortest time served averaged 6 1/2 years in Michigan.

Convicted murderers accounted for three-quarters of those released from life sentences, while rapists and robbers accounted for an additional 18 percent. Nearly 30 percent of those released with life sentences had previously served time in prison for felony convictions.

The study found that in 1983, 768 people with life sentences were released from prison in the 21 states that reported figures for the survey.

Georgia released the most people with life sentences, 239, followed by California, 117 and Ohio, 113.

According to the study, more than 93 percent of those admitted or released from state prisons three years ago were men. Male offenders were more likely than females to have violated parole, to have committed a violent crime, to have received longer sentences and to have served longer in prison.

Fifty-four percent of the offenders who entered or left one of the 30 state prison systems in 1983 were white and 45 percent were black. Whites and blacks entering prison that year received the same average sentences once differences in geographical distribution and offenses were taken into account.