The joint effects of institutional and environmental factors on college enrollments on a year-to-year basis were investigated. The study population included all public and private two- and four-year colleges and universities who completed the Higher Education General Information Survey for 1975-1976 through 1980-1981. Four major environmental factors constraining enrollments were assessed: the size of the pool of potential students, the availability of resources for individuals wishing to enroll, the level of economic wealth within a region, and the level of economic activity within the region. Five institutional factors were also evaluated: program emphasis, size of the region served, admissions selectivity, type of clientele served, and price. For public institutions, federal student aid and the levels of economic wealth and activity significantly affected enrollments. In addition, institutional selectivity and the type of clientele served, as measured by aptitude scores and the proportion of part-time students, were significant predictors of future enrollments. Idiosyncratic institutional factors seem to account for most of the variation in changes in enrollments among institutions. The enrollments of private institutions were relatively stable, and for the most part, appeared to be insulated from environmental conditions. (SW)