The influence of zopiclone and nitrazepam on arousal level, muscle strength, psychomotor performance and memory function using the photopalpebral reflex (PPR), critical flicker frequency (CFF), tapping test, pursuit rotor test (PRT), choice reaction test (CRT) and memory drum test (MDT) were investigated on the day following drug administration. After the control tests were completed at 1800 h, nine healthy male university students were given zopiclone IOrng, nitrazepam lOmg and placebo in a double-blind, crossover design at 2300h. The performance on the tests, as well as the recording of subjective assessments, were repeated at 0800, 1100 and 1400 h the next day. Both nitrazepam and zopiclone produced a prolongation of PPR latency and a decrease of CFF. The effects of zopiclone were weaker and shorter in duration than those of nitrazepam. Tapping rate, PRT and CRT improved or did not change, and there were no significant differences between the drugs. The MDT was impaired by both drugs in the morning, with nitrazepam continuing to produce memory impairment, while zopiclone did not produce further deterioration. No significant changes were observed in the subjective assessment of the subjects. These results suggest that a medium dose of zopiclone lowers arousal level and impairs memory, especially during the morning following drug administration, but produces no change in muscle strength, psychomotor performance or self-qssessment. Finally, the residual effects of zopiclone appear to be weaker than those of nitrazepam.