Haloperidol, a central nervous system dopamine blocker, was given im to seven normal volunteers at a dose level (1.0 mg) known to have central nervous system effects. Plasma PRL levels rose sharply in response to haloperidol, but plasma arginine vasopressin levels did not change significantly. These data do not support the hypothesis of a prominent dopamine neurotransmitter regulation of arginine vasopressin secretion.