The effects of the selective 5-HT2 receptor antagonists, ritanserin (1, 5 and 10 mg) and ICI 169.369 (50 and 100 mg), were studied on the sleep EEG of healthy volunteers using home-based Medilog 9000 cassette monitoring. Ritanserin (5 and 10 mg) produced a significant increase in slow wave sleep (SWS) while ICI 169,369 also increased SWS but only at a dose of 100 mg. These findings are consistent with the proposal that selective 5-HT2 receptor blockade increases SWS in humans; however, the data cannot exclude involvement of the closely related 5-HT1c receptor in this effect.