The Pentagon gave Defense Secretary Caspar W. Weinberger an official send-off yesterday, mixing politics with flamboyant military pomp and aircraft demonstrations that disrupted air traffic at National Airport for 45 minutes.

President Reagan, in his second public farewell to Weinberger in as many weeks, said the departure of the combative Pentagon chief does not mean the administration will back down from its push to strengthen the military and deploy the Strategic Defense Initiative.

"To anyone who calls for even the slightest slacking off in commitment to a strong and ready national defense, I'll only have two words: Cap Weinberger," Reagan told a crowd of about 2,000 Pentagon officials, Cabinet members and others gathered on a Pentagon parade ground yesterday morning.

Reagan's comments were sandwiched between an hour of brass bands, fifes and drums, a 19-cannon salute and formation flights of some of the military's most powerful aircraft streaking across the Washington skyline, including a B1 bomber, the plane that brought Weinberger much criticism in his final months in office because of its avionics and other problems.

The aerial demonstrations by the bomber, Army Black Hawk helicopters and formations of Navy, Marine and Air Force fighter jets forced National Airport to close its runways to incoming aircraft for 45 minutes and disrupted some air service for another 40 minutes, delaying 37 flights, according to Federal Aviation Administration spokeswoman JoAnn Sloane.

Weinberger, after almost seven years at the Pentagon and just four months short of becoming the longest-serving defense secretary in history, announced earlier this month that he is retiring because of the health of his wife, Jane, who suffers from back problems and arthritis. Frequently leaning on a cane, she watched the show yesterday from the audience and nodded thanks to her husband when he sent her a bouquet of miniature red roses at the end of the ceremony.

The man Reagan has nominated to succeed Weinberger, national security adviser Frank C. Carlucci, sat nearby. Carlucci is expected to receive Senate confirmation this week.

Reagan said yesterday he will continue to rely on Weinberger as a "trusted friend and adviser." He noted, "This old soldier, Cap Weinberger, isn't going to fade away."

He also said the administration will not dilute its support for many of the programs Weinberger championed, especially the SDI missile defense system.

"We will not unilaterally disarm in this one area or any area," Reagan said.

Weinberger, later in the ceremony, quipped that he appreciated Reagan's comments on SDI even more than the Presidential Medal of Freedom with Distinction that Reagan presented to him.

The president's remarks portrayed a different post-Weinberger Pentagon than that described to Congress last week by Carlucci when he appeared at his confirmation hearing.

Carlucci, using far more conciliatory tones than either Reagan or Weinberger, told the Senate Armed Services Committee that "we're a ways from being able to make a judgment" that SDI is cost-effective enough to justify a decision on deployment. Carlucci also indicated he would be willing to negotiate military budget compromises with Congress, including reductions in force size and some weapons purchases.

Despite the elaborately choreographed military send-off, Weinberger will remain on the job until next Monday, when Carlucci is expected to formally move into the Pentagon, according to officials.

Weinberger, saying he leaves the job with "profound regret," left the Pentagon parade ground yesterday with a chest full of distinguished service medals from the three military services. Both Weinberger and Reagan wiped tears from their eyes as the military band played the hymns of the four services.

But as the military planes roared over the closing of the ceremony, some Northern Virginia residents with a different vantage point were experiencing other emotions. Albert Forte was working at his Falls Church home when the windows began rattling violently. He looked out, saw fighter jets and a bomber streak by, and all that came to mind was war: "It was very frightening."

Staff writer David S. Hilzenrath contributed to this report.