The effects of single oral doses of zopiclone and temazepam were investigated in eight healthy male volunteers using a single blind, placebo controlled cross over study. Doses of zopiclone were 7.5 and 15 mg while the dose of temazepam was 20 mg. Each dose was separated by at least a one-week washout period. For each subject the dim light melatonin onset (DLMO) was determined on a screening night and the drugs were administered at the time of the DLMO. Melatonin concentrations were determined by radioimmunoassay from plasma samples collected throughout the night. Both temazepam and zopiclone tended to reduce the amount of melatonin secreted, as determined by the area under the plasma concentration time curve. The differences from placebo were not statistically significant (F 3.31 = 1.07, P > 0.1). Similarly a repeated measures analysis of variance on the plasma concentration-time curves did not show any statistically significant differences between drugs and placebo (F 3.28 = 1.15, P > 0.1). There was no evidence from this study of a phase shifting effect of the drugs used. The reasons for the lack of effect on melatonin may be due to the differences in potency of the interaction of these drugs with the GABA-benzodiazepine-chloride ion channel.