Baseline and post-electroconvulsive shock (ECS) prolactin levels were determined in serum samples from 13 rabbits, with and without pretreat-ment with the serotonin-2 (5-HT2) receptor blocker ketanserin. Without ketanserin, ECS did not raise serum prolactin levels over baseline. After ketanserin, baseline serum prolactin levels were 50% lower and post-ECS prolactin levels were twice this new baseline level and close to post-ECS prolactin levels without ketanserin. These observations suggest that serotonergic neurotransmission potentiates stress-induced prolactin secretion, but contributes little to ECS-induced prolactin release.