Neuroleptic-induced alterations in serum prolactin levels may have important clinical implications. Twenty-two stable schizophrenic patients, taken off medication over a 3-week period, were followed for 40 weeks or until the neuroleptic was reinstituted. Serum prolactin levels were measured during neuroleptic treatment and during the neuroleptic-free period. When serum prolactin levels were plotted over time, 55% of the subjects showed shifting prolactin levels in the shape of a "V" (i.e., a fall in prolactin level preceding an increase in the level) in the early weeks following withdrawal. Subjects with the "V" shape had significantly lower prolactin levels during neuroleptic treatment than those without the "V" shape. The pathophysiological significance of the prolactin "V" pattern is uncertain. It is consistent, however, with transient dopaminergic hyperactivity following neuroleptic withdrawal.