Little is known about children's activity levels during physical education classes and how they relate to the national health objectives for the year 2000 (U.S. Public Health Service, 1991). We systematically observed students' physical activity and associated variables in 293 third-grade physical education lessons in 95 schools in 4 Child and Adolescent Trial for Cardiovascular Health (CATCH) centers in California, Louisiana, Minnesota, and Texas. The influence of independent variables was assessed simultaneously by analysis of variance. Significant differences among study centers were evidenced for both physical activity and lesson context variables. Additionally, there were differences among variables for lesson location and teacher specialty, but not for teacher gender. Boys were more active than girls, but only during free play opportunities. The findings have widespread implications for educators responsible for developing and implementing health-related physical education programs, as well as for designing and conducting staff development.