Resumption of nesting in broody turkeys that had been temporarily deprived of nests was accompanied by an increase in levels of circulating PRL. Under such conditions, blockade of serotonin (5-HT) biosynthesis by p-chlorophenylalanine (PCPA) completely inhibited the rise in serum PRL which otherwise accompanied the return of these birds to floor pens. Even though PCPA blocked the rise in serum prolactin, it did not alter the resumption of nesting. Intraperitoneal administration of 5-hydroxytryptophan (5-HTP), the immediate precursor of 5-HT, to PCPA-pretreated turkeys restored the release of PRL. Administration of thyrotropin releasing hormone (TRH) to PCPA pretreated turkeys overcame the blocking effect of PCPA on PRL release. These data suggest that serotonergic neurons have a positive, permissive effect on PRL release in response to nesting stimulus.