ANNAPOLIS, JAN. 11 -- Seven persons were hospitalized, five in serious condition, after being overcome by chemical fumes today in their north Anne Arundel County houses.

State environmental officials ordered 26 houses evacuated in a small neighborhood next to the Baltimore Beltway as they tried to determine the chemical's origin. They said they were concentrating on a nearby pharmaceutical company where a chemical leak was reported last week.

County fire officials said they were called to an unnamed subdivision just north of Ferndale at 9:30 a.m. and found seven persons complaining of nausea, dizziness, headaches, difficulty breathing and disorientation. Five of them were in serious but stable condition tonight and being treated for chemical inhalation at the University of Maryland's Shock Trauma Center in Baltimore, an 11-year-old boy was being treated at South Baltimore General Hospital and one person had been released.

County fire officials and Red Cross volunteers set up an emergency shelter at the nearby Ferndale Fire Station.

Bob Heim, a Red Cross volunteer, said tonight that motel rooms would be found for at least one family but that most of the others have found relatives or friends with whom to stay.

"It seems like a very settled and stable neighborhood," Heim said. "Everyone's got lots of friends and relatives in the area."

Ray Feldmann, a spokesman for the Maryland Department of the Environment, said the houses will remain closed until experts are sure the fumes are gone and learn the nature of the chemical.

He said the fumes were confined to a very small neighborhood and had only been found indoors. He said victims reported that the fumes seemed to come from bathtub drains.

Feldmann said investigators are centering their initial inquiries on Kanasco Limited, a pharmaceutical company on nearby Robinwood Road where there was a recent leak of the chemical methyline chloride, which was found in the company's septic system.

Jerald J. Oppel, an attorney for Kanasco, said last night that company officials do not believe their plant is responsible for the fumes.

He said the company has not used methyline chloride since the middle of last week, and that its own tests made outside the plant this morning detected only "harmless trace amounts" of the chemical.

A spokeswoman for the Shock Trauma Center said three of those hospitalized had been treated in a hyperbaric chamber. In the chamber, commonly used to treat deep-sea divers suffering from the bends, patients are exposed to high pressures and oxygen in an effort to clear poisons from their lungs.

Those hospitalized last night came from three families, all of whom live in a small neighborhood bordered by Cedar Avenue, Locust Avenue and Cherry Lane.