Almost half of American schoolchildren have never experienced a single tooth cavity, and the rest are showing a significant decline in dental decay, according to a study released today by the National Institute of Dental Research.

The study, which involved dental examinations of about 40,000 school children aged 5 to 17 coast-to-coast, showed that 49.9 percent had teeth with absolutely no decay.And among the others, institute officials said, decayed tooth surfaces had declined by 35 to 36 percent.

Dr. James P. Carlos, chief of the institute's epidemiology branch in suburban Bethesda, Md., said that decay between the teeth, which is the most difficult to treat, "has practically been eradicated."

Speaking of tooth decay, Carlos said, "This disease, which once was probably the most chronic disease of childhood, is being decreased very fast. It is declining 35 to 36 percent every seven years and now half of the children in the country have never even experienced dental decay," he said.

Carlos attributed the decline to the increasingly widespread use of fluoride in the nation's water supply, in toothpaste and in mouthwash.