Serum prolactin (PRL) concentrations were monitored in seven normal adult men for 1 h before and 7 h after i.v. and i.m. injection of 0.25 mg and 0.50 mg haloperidol. The magnitude of the PRL response was dose-related but quite variable across subjects. The patterns of PRL secretion over the 7 h post-injection period also varied greatly among the subjects. Differences in serum heloperidol concentrations accounted for 88% of the variability in the magnitude of the PRL response to the 0.5 mg IM haloperidol dose, but only accounted for 60% of the PRL variability following the 0.5 mg IV dose. The pattern of haloperidol disappearance from serum was similar across the seven subjects and thus unrelated to the variable patterns of PRL response.