A comparative evaluation of the effectiveness of center-based, home-based and parent-based delivery systems for preschool intervention services was undertaken. Over a 5-year period, the Pennsylvania Research in Infant Development and Education Project enrolled more than 170 disadvantaged children in its two principal component programs: the center-based program and the home-based program. A third component, the parent-based program, was superimposed on the other two by involving 20 mothers of randomly chosen children who were simultaneously participating in either the center- or home-based programs. Children in all three programs were enrolled between the ages of 12 and 20 months for a 2-year period. Statistical analyses of pre- and posttest data on an extensive battery of test measures pooled over this 5-year period showed that both the center-based and home-based programs produced significant developmental gains, but that the center-based program was significantly more effective. The addition of the parent-based program did not result in further gains for children in either the center-based or home-based programs. (JMB)