The influence exerted on sleep by lormetazepam (LMZ) and ethanol (Et), either alone or in combination, was investigated in 10 healthy volunteers (mean age 22.1 years) by polysomnography (PSG). PSG was recorded on 1 adaptation night, 1 baseline night (Base-N), and 1 night under LMZ 1 mg (LMZ-N), Et (0.8 g/kg) (Et-N), and combined LMZ and Et at the above doses (Comb-N). PSG was performed during each experimental night from 23.00 until 7.30 the following morning. Sleep stages were recorded visually according to the criteria of Rechtshaffen and Kales. To determine the time periods during which the influence of LMZ, Et, and the combination of the 2 agents appeared most prominently, we divided the time from 23.00 to 7.00 into 4 2 h periods.On LMZ-N, sleep latency was reduced as compared to that on Base-N. On Comb-N, sleep latency was reduced, sleep efficiency was increased and % stage 2 increased as compared to those on Et-N. In the study of 2 h periods during each experimental night, % stage 2 was increased and % stage W decreased during the first half of sleep on LMZ-N and Comb-N.These findings suggest that LMZ 1 mg alone and the combined use of LMZ and Et caused no effect on REM sleep.