The Commerce Department, plagued by leaks of some of its most sensitive economic statistics, is moving both to reduce the possibility of their occurrence in the future and to investigate how they happened in the past.

Beginning this week, for example, the department will release the preliminary estimate of the third-quarter gross national product on Thursday morning only 15 or 16 hours after officials at its Bureau of Economic Analysis come up with the figures.

In addition, only about a dozen people at Commerce will have access to the numbers prior to their release. The chairman of the Council of Economic Advisers will continue to get the data prior to its release, Commerce officials said.

Commerce, in addition to reducing the number of persons who can see the figures, is keeping a detailed running log of exactly who does have access and when. The department also will keep a log to show if anyone leaves the BEA building at 14th and K streets NW with the figures before their release.

Meanwhile, the department's inspector general, Sherman M. Funk, is investigating leaks of GNP figures in both July and September. The FBI is assisting by conducting lie detector tests of some Commerce employes who agreed voluntarily to take them.

The leaked numbers have been given wide circulation in financial markets on the day before their official release. At least one Seattle broker has called Commerce several times to let officials there know the numbers are out, officials said.

"It's been kind of ridiculous," economist William Melton of Investors Diversified Services in Minneapolis said of the GNP figures. "The thing has been leaking like crazy."

In the past, quarterly GNP estimates have been released on the morning of the second day after their completion. Figures on personal income and personal expenditures for the third month of the quarter, which are completed along with the GNP data, have been released on the first day after completion.

This week the release dates have been reversed, with the GNP figures to be out Thursday and the less sensitive personal income data on Friday.

Advance knowledge of the GNP numbers could give a participant in financial markets an opportunity to make an investment in anticipation that the market would move in a particular way and give him a profit. Whether anyone has profited by the leaks is unclear. Inspector General Funk declined to comment on the continuing investigation.

The investigation has focused on the department itself. There has been no indication of any leak from the CEA.

Last month, when Larry Moran, a BEA public affairs official, arrived at the Council of Economic Advisers office in the Old Executive Office Building next to the White House with a copy of the so-called flash estimate of third-quarter GNP, he was told the markets already had the number.