Trustbusting activity has dropped significantly at the Justice Department since 1976, raising questions about staff morale and suggesting changes in enforcement priorities at the antitrust division, according to internal documents.

The documents, copies of which were obtained by The Washington Post, were prepared in connection with meetings held by antitrust officials to discuss lagging staff productivity.

In fact, the division filed about 34 percent fewer cases between 1976 and 1978 than during the previous period and this could become the division's least productive, in terms of case filings, since 1970.

"It's getting to be kind of a desperate situation over here for some of the line attorneys," one official said. "They feel strongly that there is no enthusiasm for their work."

Although antitrust officials point out that division lawyers and economists have been increasingly spending their time filing comments before regulatory agencies and commenting on legislation, they also say they cannot precisely explain the decline in case filings.

Since the figures were compiled last summer, the response of division officials has been to institute systems to log the time spent by agency professionals and paraprofessionals on antitrust activities that do not involve litigation.

"Intervening at places like the Interstate Commerce Commission is just about as time consuming and complex as taking a case to District Court," said Frank V. Battle, the division's executive officer.

Battle said the division had not been able to chart the time the staff had spent on these activities, giving the division little opportunity to explain its change in activities.

Division officials also suggest that the decline in case filings might be attributed to cuts in staff assigned to case development and prosecution, increasing use of litigating attorneys to handle Freedom of Information requests and congressional queries, and increasing, time consuming prosecution of so-called "big cases."

Further, they suggest the decreases might be a result of recent court decisions that make it more difficult to prosecute violators of merger laws, decreasing cooperation by companies during investigations and an increasing sophistication among members of the antitrust bar.

On the other hand, some federal antitrust officials and other observers attribute the decline in case filings to a cautiousness and conservatism within the antitrust division's management.

"Many people have felt for a long time that the front office crew is so conservative they would beat cases to death," one federal official said.

Mark Green, the director of Congress Watch, the lobbying group associated with Ralph Nader calls the decline a "distress signal."

Green said in a recent speech that the division under Assistant Attorney General John H. Shenefield and his recent predecessors has become more of a "think tank" than a law enforcement operation.

Shenefield, who is now serving as acting associate attorney general, did not return several telephone calls this week. Shenefield is expected to be nominated to that job when his successor at the division is named.

Whatever the precise reasons, six of the antitrust division's regional offices experienced a drop in case filings, according to figures contained in department memos.

More startling is the fact that the number of cases filed by the division's Washington office dropped from 106 to 54, when the 1972- 1975 period is compared with the slightly shorter time between 1976 and 1978.

In addition, between 1976 and 1978 the division filed 27 merger cases, down from 58 filed between 1965 and 1968 and 45 between 1972 and 1975.