RP 62203, a naphtosultam derivative, is an antagonist at the 5-hydroxytryptamine2 (HT2) receptor. The sleep pattern of rats treated orally with RP 62203 was studied at doses ranging from 0.5 to 4 mg/kg. Following RP 62203 administration, the duration of deep nonrapid eye movement (NREM) sleep was found to increase at the expense of wakefulness in a dose-dependent manner from 0.5 mg/kg. The 5-HT2 receptor agonist DOI and the 5-HT1a receptor agonist 8 OH-DPAT induced a dose-related increase in wakefulness; treatment with RP 62203 reversed the enhancement of wakefulness produced by DOI but not that produced by 8 OH-DPAT. These data provide further evidence for the involvement of 5-HT2 receptors in the regulation of NREM sleep in rats. RP 62203 could therefore be of clinical interest in the management of sleep disorders, particularly those developing within a psychiatric context.