To determine whether there is an association between prolactin (PRL) levels and psychopathology or tardive dyskinesia during neuroleptic treatment, we measured plasma prolactin levels and neuroleptic activity (NA) in 33 chronically treated male schizophrenics. Neuroleptic dose, plasma NA, and PRL were significantly intercorrelated. Plasma PRL levels were also measured in 8 male schizophrenics recently withdrawn from neuroleptics and in 18 normal male controls. In treated patients, but not in controls, PRL levels decreased with age and duration of illness, two variables that we interpreted as indirect measures of neuroleptic exposure. PRL levels in patients recently withdrawn from neuroleptics were lower than in treated patients or controls, which was suggestive of rebound hypoprolactinemia. A prolactin index, calculated as the ratio of PRL levels to NA, was inversely correlated with paranoid symptoms and tardive dyskinesia in younger treated patients. These results lead to speculation that tuberoinfundibular dopaminergic supersensitivity develops in chronically treated schizophrenics and that it is associated with nigrostriatal supersensitivity, manifested by tardive dyskinesia, and paranoid symptoms, which may reflect mesolimbic supersensitivity.