This exploratory study clarifies the goals and functions of long day and short day preschools, as related to the requirements of the client groups being served, and analyzes their effects on children's socialization and elementary school achievement. It was hypothesized that low socioeconomic status children who had attended long day preschools would be more likely to meet the normative expectations of the primary school than low socioeconomic status children who had attended short day preschools. High socioeconomic status children who had attended long day preschools were expected to do about equally well as those who had attended short day preschools or no preschools at all. Data were obtained from classroom observations and interviews with 21 teachers in six nursery schools and day care centers. School records of 584 second grade students were analyzed. The measures of school achievement used were: (1) ability to achieve normal promotion, (2) the absence of teacher comments indicating unacceptable classroom behavior, and (3) scores on two objective achievement tests, scored by an outside agency. The central hypotheses were largely supported by the findings. (Author/NH)