In conscious rats, serotonin microinjected into the basal hypothalamus caused secretion of GH maximal within 10-25 min. The effects of serotonin on GH were blocked by the non-selective serotonin receptor blocker, metergoline 2.5 mg/kg, but not by the serotonin type 2 receptor blocker, ketanserine 0.2 mg/kg. Injections of serotonin in the preoptic/anterior hypothalamic area were without effect. It is concluded that activation of serotonin receptors, probably type I, on or near GH releasing factor neurons in the arcuate nucleus causes secretion of GH and that serotonin has no direct effect on or near somatostatin neurons in the preoptic anterior hypothalamic area.