Zopiclone is a new short half-life cyclopyrrolone hypnotic agent acting at the GABA-benzodiazepine receptor complex. In order to characterize its pharmacological profile, the effects of 7.5 mg zopiclone on nocturnal melatonin secretion were investigated under polysomnographic control in 11 healthy subjects following acute and subchronic administration as well as after abrupt discontinuation of the drug. No effect of zopiclone on the melatonin plasma levels could be observed. Regarding both total melatonin production and the temporal pattern of melatonin secretion during the night, there was no difference between placebo baseline condition, acute and subchronic administration, and discontinuation. In contrast, the sleep EEG data demonstrated the hypnotic efficacy of zopiclone under acute administration and indicated a rebound insomnia after abrupt discontinuation. Moreover, alterations of sleep architecture were found under treatment as well as after discontinuation. Whereas, with regard to sleep EEG parameters, zopiclone appears to be comparable with some short-acting benzodiazepines, a discrepancy between the missing effect of zopiclone on pineal function and the suppressing influence of benzodiazepines known from the literature becomes obvious. The fact that zopiclone does not interfere with nocturnal melatonin secretion at pharmacologically active doses as indicated by alterations in sleep EEG parameters might possibly point to a pharmacodynamic difference between the two drug classes.