WASHINGTON--President Reagan declared yesterday that U.S. military forces have been told to "defend yourselves" as the death toll from the Iraqi attack on a Navy frigate mounted to 37.

Reagan, in a speech to graduating high school seniors from the Chattanooga, Tenn., area said "this tragedy must never happen again," and Defense Secretary Caspar W. Weinberger acknowledged that "we don't know why" the USS Stark did not return fire on the Iraqi warplane that attacked it in the Persian Gulf.

In an interview with Chattanooga area newspapers, Reagan said the administration is waiting to find out why the guided-missile frigate didn't return fire at the Iraqi missilefiring plane in the 60 to 90 seconds such a response would have been possible.

"What we're waiting to find out now is what exactly was the situation on the ship and the attitude, and why they ... hadn't prepared," Reagan said. He noted that "general quarters hadn't been sounded, as it might be, if a hostile plane were coming into the area."

The president, however, did say the United States "had a very fulsome apology" from President Saddam Hussein of Iraq.

"The whole thing, the course of the plane coming down that coast was the course that's taken by Iraqi planes all the time, and they're never, we've never considered them hostile at all," Reagan said. "They've never been in any way hostile. And this was at night, of course, so never had any visual sight of the target. They fired that missile by radar."

"We're going to do what has to be done to keep the Persian Gulf open," Reagan said. "It's international waters. No country there has a right to try and close it off and take it for itself. And the villain in the piece really is Iran. And so they're delighted with what has just happened."

Rear Adm. Harold J. Bernsen, speaking to reporters in Bahrain, said the Stark had about a minute's warning that an Iraqi warplane had turned on its weapons radar and had locked in on the frigate. But based on preliminary reports, the admiral said, there was "no indication" to the crew that a missile had actually been launched.

Reagan said, "Our ships are deployed in the gulf in order to protect U.S. interests and maintain free access and maintain freedom of navigation and access to the area's oil supplies. It is a vital mission, but our ships need to protect themselves and they will."

From now on, the president said, "if aircraft approach any of our ships in a way that appears hostile, there is one order of battle. Defend yourselves. Defend American lives."

A memorial service for the victims will be held Friday at the Mayport Naval Base in Florida.

In fast-moving developments as the nation's capital reacted to the heaviest loss of American lives since the bombing of U.S. Marine barracks in Beirut, Lebanon:

.Senate Republican leader Bob Dole of Kansas questioned the policy which has sent American ships into the midst of the Iran-Iraq war. "We need to rethink exactly what it is we are doing in the Persian Gulf. What are our goals? What is our strategy? What are the risks? And how much cost are we willing to pay?"

.The State Department said it was willing to join with Iraq in conducting a joint investigation into the attack on the Stark.

.The Administration said that despite the Iraqi attack on an American frigate, "general agreement" has been reached with Kuwait to put American flags and American captains aboard Kuwaiti oil tankers operating in the gulf. The purpose is to protect the shipping lanes of the Straits of Hormuz.

.Weinberger, testifying before a Senate committee, said the Stark should have had enough warning time to engage its defenses. "They were not used. We don't know why."

The secretary said the Stark carried a crew of 222. The Pentagon said the death toll had reached 37, and Weinberger said the 185 others "are fine." But he did not elaborate.

As for the Stark itself, the Navy said the 453-ft. ship--with a gaping hole in its left side and extensive fire damage inside--had been towed by the destroyer Conyngham to the Sitra Anchorage off the Bahrainian capital of Manama.

A team of technicians was dispatched earlier in the day from Norfolk, Va., to Bahrain to conduct an initial damage assessment and begin temporary repairs, the Navy added.

The Navy said the ship's crew, with assistance from other U.S. warships, had finally managed to douse all the fires that were touched off when at least one Exocet missile slammed through the frigate's hull in the attack Sunday night.