A longitudinal data set is employed to explore the sources of stability and change in young adults' health beliefs and behavior concerning drinking, diet, exercise, and wearing seat belts. There is substantial change in the performance of health behaviors during the first three years of college, and peers have a strong impact on the magnitude of that change. In total, however, parents are much more important than peers as sources of influence over these beliefs and behaviors. Of the various social influence processes considered, the direct modeling of behavior appears to be the most important avenue of influence for both parents and peers. These data, along with previous papers in our research program, suggest a pattern of gradually increasing parental influence on their children's health beliefs and behavior while the children are living at home, and the persistence of that influence at least through the college years.