Longitudinal studies are presented in four schizophrenic patients to investigate further the relationship between neuroleptic treatment (haloperidol or droperidol), plasma prolactin response, suppression of neuroleptic-induced prolactin elevation by dopamine infusion, and psychopathological change. Several unexpected findings related to neuroleptic-induced prolactin response and its suppression by dopamine infusion have been observed. Data appear to open up new vistas for the validity of the 'prolactin model' in studying the pharmacodynamics of neuroleptic drugs.