The mornings along the Litoral Highway belong to the guerrillas, high noon to the Army and sundown to the guerrillas once again.

Scenes along the stretch of road running east from the blown-apart Golden Bridge through the department of Usulutan suggest the difficulties facing the U.S.-backed junta in San Salvador as it attempts to keep control of the countryside as well as the capital.

At 8 a.m., a squad of five guerrillas was openly, even casually stopping all traffic at Kilometer 83, 50 miles from the capital on this main national highway. They were accepting whatever donations anyone wanted to give.

About 30 miles farther east, just to the east of the provincial capital, Usulutan, two sabotaged tractor-trailer trucks carrying soda pop were still burning in the middle of the road. Guerrillas set them afire yesterday at 4 p.m. Hundreds of elated peasants were carrying cases of yellow and orange soft drinks on their heads through the cotton fields.

Hernan Palacios, an employe of the Miguelena bottling company who was sent out to try to salvage something, shrugged and told anyone who bothered to ask that they could take what they wanted.

Palacios said he and an associate had asked the local military base for help in protecting the shipment, but they were told the Army could not send anyone out until later because it could not risk a shootout.

"The government just doesn't have the reach. It can't do anything to protect our production or guarantee our safety. That's demonstrated right here," Palacios said as he looked at the smouldering remains of the trucks and the 2,000 fast-disappearing cases of drink.

"At least they'll come back to buy more later," he sighed.

"In the cities we don't see much," Palacios said. "Here in the countryside you see right away how much help we need." Palacios wondered if the United States finally would have to send troops to the aid of this government--a question more and more Salvadorans seem to be asking. He said he hoped not. "The violence would grow," he said. "The other countries would send more aid to the left."

An associate of Palacios, standing nearby, said all he expected of the future is "chaos. This has to end in chaos."

Yet there is now an odd kind of routine to life in this area, one to which regular travelers seem accustomed. At Kilometer 83, the squad of guerrillas stopped every car, bus and truck. Three of the young guerrillas were armed with American-made M16 rifles. One had an old M1 and the fifth a Belgian-made Fal automatic. Their guns, they said, had been "requisitioned" from "the enemy."

Yesterday there was a shootout here in which the guerrillas said that three of their group were killed and two wounded. But this morning at 6, guerrillas were back in the same spot, seeming perfectly relaxed.

Government soldiers guarding the rickety railroad bridge that provides the only means of crossing the Lempa River--the highway span was blown up in October--told travelers exactly where the guerrillas were. The leftists were not making themselves difficult to find because they knew the troops would not come out in a small group and any large movement would give them plenty of time to disappear into the bush.

The young Army recruits manning the west end of the bridge this morning did not seem, in any case, in any shape to go guerrilla hunting. One of them half stumbled across the dusty road, his eyes red and glazed, his speech slurred and incoherent. His comrades, all teen-age soldiers, giggled.

The guerrillas at Kilometer 83 were also almost children, their faces indistinguishable from those of the soldiers. But the guerrillas seemed absorbed in the job at hand. Mostly illiterate, they nevertheless talked automatically and easily of repression and liberation, using the rote language of revolution.

The oldest, the head of the little squad, was 19. Wearing a ragged, olive-green uniform, civilian shoes and bright red socks, with a battered straw hat on his head, he said he had been fighting for two years, since "I comprehended the necessity for the struggle."

A 16-year-old said he had been in the ranks of the revolutionary Workers and Peasants Party and now the Farabundo Marti National Liberation Front for almost two years also. His brother was a guerrilla, he said, and was killed in May.

A bus full of peasants stopped at the guerrillas' signal. A couple of the insurgents walked beneath the open windows. Some passengers, and the driver, threw coins and small bills.

And if there is no contribution? "We just let them go," said one of the guerrillas.

Later in the day, along the road back to San Salvador, other buses were being stopped. This time, government soldiers were pulling everyone out and lining up the men to be searched for weapons. No money was asked or offered. The peasants endured the search in silence, showing no particular fear or irritation at the inconvenience. It was just another stop on the highway, part of the war's new and quickening routine.