Ritanserin, 5 mg, produced a substantially larger increase in SWS (51.4%) than either ketanserin, 20 mg (17.2%) or ketanserin, 40 mg (24.4%). Ritanserin has a significantly higher affinity than ketanserin for 5-HT2C receptor binding sites in the human brain and, based on estimates of per cent occupancy by the two compounds at brain 5-HT2A and 5-HT2C receptors, we conclude that SWS in humans is primarily regulated by 5-HT2C receptors.