President Reagan today left open the possibility that he will cancel a planned stopover in the Philippines in November in the aftermath of the assassination of opposition leader Benigno Aquino Jr.

Administration officials, who asked not to be identified, said that the United States issued a strong statement after the assassination in hopes that it would produce a swift and credible investigation by the government of President Ferdinand Marcos of the shooting.

The officials said that the decision on whether Reagan should make the trip will be made after the investigation is under way and the impact of the murder on the Philippines domestic scene is clearer.

Reagan was asked about the visit today as he left a doctor's appointment, during which he had his hearing examined. Asked if he will go, Reagan suggested that the one-day Manila stopover on his Asia trip in November is not a foregone conclusion.

"Well, I haven't had an opportunity to talk with or know the details of this, or talk with the State Department," he said. "But I'm sure we'll be making a decision on this soon." Asked whether he was worried about making such a trip, Reagan replied, "No."

While the president seemed to leave open the possibility of canceling the trip, White House spokesman Larry Speakes insisted later in the day that it still is scheduled.

"The president of the United States is thinking he's going to the Philippines. He knows he's going to the Philippines," Speakes said. He called any suggestion that the trip might be canceled "speculation."

An informed administration official, however, said that while the trip still is scheduled, "no one would hesitate to reconsider" if the situation deteriorates in the weeks ahead.

"The trip is on, and that's all that can be said," the official said. But he added that reconsideration "depends on the situation in the Philippines" in the days and weeks ahead.

Specifically, the official said that the trip depends on whether the president's physical security can be assured, whether the Marcos government is conducting a thorough investigation and whether there is domestic unrest.

The White House issued what another official called "an unusually strong" statement responding to the assassination, partly in an attempt to spur the Marcos regime into a swift but credible investigation.

"We've got the heat turned up," the official said.

Speakes told reporters here today that Reagan will not go to the Philippines if his physical security would be endangered.

"We would not put the president in a situation where we had any reservations about his security," he said.

Speakes said there is no direct link between the investigation into the Aquino assassination and whether Reagan visits the Philippines. But he said, "We trust the government of the Philippines will swiftly track down the perpetrators of this political assassination and bring them to justice and punish them."

Speakes said that "perpetrators"--suggesting that there was a conspiracy to kill the opposition leader--was used by the Philippines government. "We note that President Marcos has stated all the resources and powers of his government will be deployed in the quickest possible time to find the perpetrators," Speakes said.

He said that the U.S. Embassy in Manila had reported to Washington today that the security guards accompanying Aquino did not assassinate him. Reports that they had fired on Aquino were the result of "confused impressions" at the time, Speakes said.

Asked whether the United States would support an outside investigation into the murder, Speakes said, "It's being handled by the Philippine government."