The Drug Enforcement Administration has grown so secretive under President Reagan that Comptroller General Charles A. Bowsher has told the White House he may be forced to take the agency to court.

In a letter to Reagan last week, Bowsher outlined the frustrations that his watchdogs have encountered in trying to review DEA's efforts to attack bigtime drug traffickers.

Under a two-year-old law aimed at such recalcitrance, Bowsher and his General Accounting Office can seek a federal court order for the records in question 20 days after the White House has been notified of an impasse.

Bowsher said in his March 31 letter to the president that he hoped the access problem could be resolved before then "in a manner that will allow us to fulfill our reporting and oversight responsibilities to the Congress."

He also noted that Attorney General William French Smith had yet to make any written response to a March 1 protest to the Justice Department.

The attorney general's inaction prompted Bowsher's second-stage letter to Reagan.

Copies of both letters were obtained by The Washington Post.

The two-page "report" to the president represented the first time that GAO has gone that far under the 1980 law.

In all other cases, and they have been rare, GAO officials say, disputes have been resolved with an initial "demand letter" such as the one sent Smith.

According to Bowsher, the same kind of DEA documents at issue here have "up until recent months" been made available to GAO for previous audits. The information, he added, has always been properly safeguarded.

The study of DEA's practices had been requested by Sen. Joseph R. Biden Jr. (D-Del.), the ranking minority member of the Senate Judiciary Committee.

He wanted to know how DEA establishes who the biggest drug traffickers are, how it targets them for investigation, what kinds of investigative methods are used, and related questions.

DEA field offices in Los Angeles, San Diego, New York and Boston were selected for detailed study. Bowsher's agency asked for access to all DEA case files and other records including intelligence and enforcement reports identifying major violators as targets, work progress reports including reports of any "deviations from planned objectives," and similar data.

" W e will, in accordance with our usual practice, take all steps necessary to guard against unauthorized disclosure of sensitive or confidential information," Bowsher said in his March 1 letter to the attorney general.

Despite that assurance, the comptroller general said his office had received no formal reply as of March 31 and "no assurances that GAO would be provided access to the requested records."

In his letter to the president, Bowsher reported that "for the most part, access to the records was denied altogether" by DEA. "Although some records were provided, access to them was delayed, and not all of the records were complete."

GAO had already conducted discussions "over an extended period of time with various DEA officials, including the acting administrator," but was unable to resolve the problems at those levels.

Under the 1980 law Bowsher invoked, the president or the director of the Office of Management and Budget can forestall court action without producing records, but only if the data relate to "foreign intelligence or foreign counterintelligence activities" or if disclosure of the documents would "substantially impair the operations of the federal government."