The effects of five environmental and five institutional factors on college and university enrollments during the period between 1975-1976 and 1980-1981 were studied with a sample of 2,101 institutions. The environmental factors were federal student aid, state student aid, number of 18-year-olds, unemployment, and level of economic wealth as characterized by average weekly earnings. The institutional factors were selected to represent the manner in which colleges and universities have positioned themselves within their environment. The institutional factors were: percentage of total students enrolled on a part-time basis, tuition and fees, admissions selectivity, percent of in-state students, and program emphasis as characterized by percent of degrees awarded in the humanities, social sciences, and education. A lagged enrollment variable was included to control for differences in institutional size and to study the effect of the direction and magnitude of past enrollment change on current enrollments. A pooled cross-sectional time series regression design was used to analyze the data. Separate regressions were run for public and private two- and four-year colleges and for major doctoral, comprehensive, and general baccalaureate institutions. (SW)