As part of an ongoing longitudinal study of drug-free schizophrenic patients, we serially sampled resting early morning prolactin levels in 10 subjects. In a preliminary analysis, these levels were compared to those found in matched normal control subjects over a 4-year period. Both control and schizophrenic subjects showed a marked annual variation in prolactin levels. Six schizophrenic patients sampled in each quarter of the year showed a significant annual rhythm, with prolactin highest in the spring (March-May). In seven schizophrenic patients and nine controls sampled at two seasons in the year, prolactin was significantly higher in spring-summer (March-August) than in fall-winter (September-February), with no difference between patients and control subjects.