About half of American children and adolescents aren't getting enough exercise to develop healthy hearts and lungs, concludes a two-year federal study released yesterday.

The study, funded by the U.S. Public Health Service, surveyed 8,800 students in grades 5 through 12 across the nation. It surveyed fitness and exercise habits and also put students through rigorous physical tests designed to measure overall health and fitness, rather than athletic ability.

"This study should serve as a warning," Health and Human Services Secretary Margaret M. Heckler said in issuing the report. "It shows that America's school children are not achieving the lifetime fitness skills required to promote good health."

The study, the first of its kind in nearly a decade, found that only 36 percent of students take physical education classes daily, compared with the Public Health Service's 1990 goal of 60 percent. Secondary school programs tend to focus on team sports rather than individual and lifetime skills for promoting health, it concluded.

The study also found that young Americans have grown fatter since the 1960s, but researchers cautioned that further study is needed to determine whether this higher average body fat is a health problem.