The Reagan administration is considering a change in regulations that would permit U.S. military advisers in El Salvador to take their M16 automatic rifles into the field under certain circumstances, according to administration sources.

Although officials indicated that the matter was not decided, they said some change was expected very soon, perhaps a regulation using vague language that, for example, would allow advisers to keep "personal" weapons such as an M16 with them under specific situations.

The new look at regulations that currently limit U.S. advisers to carrying only sidearms comes in the aftermath of a controversial episode that unfolded in El Salvador last week and which also has caused some behind-the-scenes controversy in Washington.

In that episode, the American ambassador in El Salvador, Deane R. Hinton, ordered a U.S. Army lieutenant colonel to leave the country after the officer and two other junior officers were photographed by a television crew carrying their M16s into an area contested by leftist guerrillas.

The advisers were supervising construction of a pre-fabricated bridge to replace one blown up by guerrillas fighting the U.S.-backed government.

Although Hinton was acting under regulations which currently prohibit the carrying of combat weapons by American advisers, sources here say the ambassador's action, which one official characterized as an "over-reaction," annoyed the Pentagon and also caused concern within the State Department and the White House. The concern in Washington, the official explained, was how the public here would react if a GI helping to build a bridge in El Salvador is killed by guerrillas in an area where he was not supposed to be carrying a weapon to protect himself.

The situation illustrates a dilemma for the administration that is deepening as the American involvement with the beseiged government of El Salvador grows.

When the United States began to step up its military aid to El Salvador a year ago and began sending additional teams of advisers, the administration went to great lengths to try to allay public and congressional fears that this increased involvement was similar to the way American involvement began in Vietnam.

The administration stressed that the U.S. military personnel would be trainers and not "combat advisers," that they would remain in garrisons or otherwise safe zones, would carry only sidearms and would not go into combat zones.

As the war has expanded, however, and as the territory that guerrillas are contesting has grown, it is far less clear what a combat zone is.

The area where the officers were heading last week was one Hinton described later as "reasonably safe," yet it obviously was also an area where guerrillas had blown up a bridge.

Also, as the scope of the American involvement has expanded and the complexity of military equipment there has increased, advisers have had to do more traveling in the country.

Officials here said yesterday that they expected some recommendation from Hinton to be arriving in Washington momentarily on possible changes in the regulations.

The ambassador, who had the authority to expel the Army officer, is said to have come under considerable pressure from the administration, through the State department, to change his stance on this matter.