Deputy Secretary of Defense Paul Thayer yesterday said the Pentagon must buy weapons now and test them later to ensure that its arsenal remains up-to-date.

Thayer was responding to recent charges that the Defense Department buys billions of dollars worth of weapons without knowing whether they work.

He acknowledged that there is "concurrency" in testing and billion-dollar purchases, but said that overlap is necessary and inevitable.

"The old myth of 'fly before buy' is just that, it's a myth," Thayer said at a Pentagon news conference. "If in fact you did wait until such time as development testing and operational testing was complete before going into large-scale production, in many cases by the time the weapons system got into the hands of the user it would be obsolete."

"Fly before buy" was a phrase coined by Melvin R. Laird, secretary of defense under President Nixon, to indicate that the Pentagon would not buy weapons before they had been tested thoroughly.

The General Accounting Office, investigative arm of Congress, recently said it had examined 10 weapons systems costing $33 billion that the Defense Department ordered without knowing whether they would be effective.

The report also said Pentagon officials have "conflicting interests" when they are charged with both developing a new system and seeing whether it works.

Thayer, a former aerospace executive, said he disagreed with the GAO's conclusions.

"It wouldn't be the first report we ignored," he said.

The deputy secretary also rejected a proposal by Sen. David H. Pryor (D-Ark.) to establish an independent testing office in the Pentagon. He said the Pentagon has "nothing to be ashamed of" in weapon testing, and anyone who says otherwise is "misinformed" or "irresponsible."

"The fear I have," Pryor responded, "is Mr. Thayer is developing the Pentagon mentality of rushing in to purchase weapons and manufacture weapons before they are ready . . . . In his heart, I imagine, he knows he's wrong."