The neuronal mechanisms that govern prolactin (PRL) secretion in the turkey appear to involve monoaminergic systems. Considerable evidence indicates that serotonin (5-HT), acting centrally, is a potent stimulator of PRL secretion. This study, using birds actively immunized against VIP, tests the hypothesis that 5-HT stimulates PRL secretion by releasing vasoactive intestinal peptide (VIP). Nonimmunized turkeys were injected ip with saline, quipazine (5-HT agonist; 5 mg/kg), methysergide (5-HT antagonist; 8 mg/kg), or methysergide plus quipazine, and VIP-immunized birds were injected with saline or quipazine. Quipazine increased plasma PRL levels from 26.8 ± 7.1 ng/ml at Time 0 to a peak value of 148.1 ± 31.4 ng/ml 2 hr after injection. Pretreatment with methysergide or VIP-immunoneutralization abolished the PRL response to quipazine. Intraventricular infusion of 5-HT (1 nmol/min) caused plasma PRL to rise from a baseline of 16.3 ± 2.6 ng/ml to 85.2 ± 14.3 ng/ml after 30 min in nonimmunized control birds. Serotonin infusion did not induce PRL secretion in the VIP immunized birds. These findings suggest that serotonergic stimulation of PRL secretion in the female turkey requires a functional VIPergic system.