Several steroid compounds affect neuronal function, primarily by modulating the GABAA receptor complex. A circadian variation in the brain concentration of neurosteroids has been reported in rats and humans. We have previously reported that natural occurring or synthetic neuroactive steroids such as androsterone and alphaxalone also have a rhythmic effect on behavior (anesthetic and anticonvulsant activity) and GABAergic activity. In the present work, we have tested the ability of neuroactive steroids to phase shift circadian rhythms in hamsters. The GABA(A) negative modulator dehydroepiandrosterone sulphate (DHEAS) elicited phase advances when administered at CT 6, while the positive modulator androsterone lacked any effect at this time. A complete phase response curve for DHEAS revealed a nonphotic-like effect. DHEAS also blocked the circadian effects of light, while androsterone induced photic-like responses. There is also evidence that neurosteroids may be present and even synthesized in the SCN. Collectively, the results so far indicate that some neuroactive steroids might modulate the activity of the circadian clock.