The effects of several serotonin acting drugs on serum prolactin levels were studied in male rats. Administration of the serotonin precursor, 5-hydroxytryptophan (5-HTP; 250 mg/kg ip) or pargyline (5 mg/kg ip) with 5-HTP (100 mg/kg ip) significantly increased serum prolactin concentration. Pretreatment of rats with the serotonin agonist, quipazine (10 mg/kg ip) caused increase in serum prolactin levels. Depletion of serotonin stores by pretreatment with p-chloro-phenylalanine methyl-ester (PCPA; 3 x 300 mg/kg ip) significantly prevent the effect of quipazine on prolactin levels. Serotonin uptake blocker, fluoxetine (10 mg/kg ip) produces an increase in the prolactin concentrations of the serum. Administration of the reserpine (2.5 mg/kg sc) did not affect serum prolactin levels. Both serotonin receptor blockers, cyproheptadine (0.5 mg/kg ip) and pizotifen (1 mg/kg ip) caused a week decrease in the serum prolactin levels. Significant changes in concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brain of rats pretreated with above serotonin-active drugs were found. These results indicate that serotonergic system can modify secretion of prolactin.