Serious efforts have been made to develop anxiolytics with improved clinical utility and reduced side effects. 5-Hydroxytryptamine (5-HT) 2C receptor antagonists are potential anxiolytics; however, their effects on vigilance are not well characterized. To compare the effects of benzodiazepines and subtype-selective 5-HT 2C receptor antagonists on anxiety, vigilance, and electroencephalogram (EEG) power density, social interaction test and polygraphic recordings were performed in male Sprague-Dawley rats after chlordiazepoxide (CDP; 4.0 mg/kg i.p.) and SB-242084 (6-chloro-5-methyl-1-[2-(2-methylpyrid-3-yloxy)-pyrid-5-yl carbamoyl] indoline) (0.1, 0.3, and 1.0 mg/kg i.p.) treatment. CDP and SB-242084 (0.3 and 1.0 mg/kg) had similar anxiolytic effects. Spectral analysis of EEG in wakefulness (W) and paradoxical sleep (PS) showed an opposite effect on activity (5-9 Hz); it decreased after CDP, whereas it increased after SB-242084 (even at 0.1 mg/kg). In addition, CDP significantly decreased slow-wave activity (0.5-4 Hz) in deep slow-wave sleep (SWS-2) and increased power at frequencies above 12 Hz mainly in W and PS. A markedly increased intermediate stage of sleep was also found after CDP treatment. At the highest dose, SB-242084 increased W and decreased SWS-2. In summary, low but potent anxiolytic doses of the subtype-selective 5-HT 2C receptor antagonist SB-242084 did not affect vigilance states but caused an increased activity in W, raising the possibility of a cognitive-enhancing effect of the drug. In contrast, acute CDP administration, based on spectral analysis of the EEG, produced a more superficial sleep along with a decreased activity.Efforts to improve the clinical utility and reduce the side effects of classic benzodiazepines have a long history (Haefely et al., 1992). Although the anxiolytic properties of these drugs are well established both clinically and experimentally (Argyropoulos and Nutt, 1999;File and Seth, 2003), the side effects like impaired short-term memory, vigilance, or psychomotor performance limit their clinical applicability (Kunsman et al., 1992;van Laar et al., 2001;Buffett-Jerrott and Stewart, 2002). On the other hand, these effects are often subtle when low doses are involved (Kunsman et al., 1992), and tolerance develops to the sedative and psychomotor effects with their continued use as anxiolytics (Lucki et al., 1986).Pharmacological characterization of the effects of the