The effects of diazepam (DZ) (0.3–1.5 mg/kg, i.p.) on sleep, cisternal cerebrospinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), and rectal temperature of cats were examined. The results showed that administration of DZ produced a significant increase (p = 0.02) in slow-wave sleep (SWS) with a peak occurring at a dose of 0.9 mg/kg. Further increase in doses of DZ decreased SWS. DZ administration produced no change in paradoxical sleep, rectal temperature, 5-HIAA or HVA CSF levels. Lack of correlation between various doses of DZ, and 5-HIAA or HVA concentrations in the presence of an increased percentage of SWS suggests a possible mode of DZ action mediated through a mechanism independent of monoamines. Usually monoamines are associated with normal sleep.