A plant nursery fire that sent stocks of potent pesticides and herbicides up in smoke forced the early-morning evacuation of 125 people in the Brookeville area of Montgomery County yesterday and left four firefighters hospitalized with respiratory problems.

Forty-four other firefighters and nine residents, some of them displaying headaches and nausea believed caused by exposure to poisonous fumes, were examined at county hospitals, treated if necessary and released. Other area residents, who had been routed from their homes about 1 a.m., were able to return home at about 5 a.m. after the fire had died down.

The fire broke out at about 11 p.m. Saturday at the Brookeville Plant Farm, 3901 Gregg Rd., gutted three greenhouses and a potting facility, and caused damage initially estimated at $500,000. Cause of the blaze is still under investigation.

Fire officials had called for the precautionary evacuation because of concern over unknown quantities of a least eight poisonous substances stored there.

Warren Isman, director of the county's Fire & Rescue Services, said the liquid herbicide Paraquat and Aldicarb, an insect killer in powder form, had caused the most serious concern. Consumed orally, both can cause lung disorders. Paraquat also can cause vomiting, diarrhea and, in high enough doses, death.

Fire officials believed most of the toxic substances were neutralized by the flames themselves. But some residents expressed concern about possible fallout onto surrounding land, includes a cattle ranch.

Isman said that firefighters on the scene had decided not to use water, so as to avoid spreading poisonous substances into the ground and contaminating the area. "The technique for handling a pesticide fire is to let it burn," he said. Confident the flames would not reach nearby propane tanks, the firefighters allowed the fire to burn itself out.

One firefighter got too close to the fumes, lost consciousness and went into convulsions, Isman said. Others had trouble breathing and began to suffer headaches and nausea.

Three firefighters were being kept under observation in the intensive care unit of Montgomery General Hospital yesterday. A hospital spokesman described their condition as satisfactory and said they were suffering respiratory discomfort. A fourth man was hospitalized in a regular ward.

When the 125 people were evacuated to a nearby church, tea and coffee were provided as fire officials briefed the crowd on the problem. "We all sat there wondering when the symptoms [of pesticide poisoning] were going to develop," joked Joanne Stang, whose home stands a few yards from the blaze. "When is the headache going to start, when will I start getting nauseous?"

One woman who had driven in a closed car past the fire began wondering aloud if she should go to the hospital, Stang recalled.

However, most people returned to the neighborhood about 5 a.m. carrying dogs, half-read books and blankets clutched in the early-morning rush. And by yesterday afternoon they were joking about the night's experience.