The purpose of the study was to ascertain whether the new hypnotic, zopiclone, was likely to produce rebound problems after short-term use, in comparison with placebo and a standard hypnotic, temazepam, and whether tapering the dosage lessened any such effects. Ten normal v olunteer subjects were administered 5 treatment sequences, each lasting 4 weeks, using a balanced design, with at least 2 weeks between sequences. The treatment sequences were: (table: see text) Each drug was given at night before retiring to bed. Daily ratings comprised a Sleep Questionnaire, Mood Rating Scales, the Spielberger State Anxiety Inventory and Bodily Symptom Scales. Both drugs improved quality of sleep but their discontinuation was followed by some worsening which was postponed but not avoided by halving the dosage for a week. Speed of, and feeling on, awakening showed discontinuation effects with temazepam but not with zopiclone. Zopiclone was associated with feelings of being troubled, tense, antagonistic and bored whereas temazepam produced drowsiness, clumsiness, dreaminess and sadness. Some increase in these ratings was noted after stopping temazepam and these were less after having the dosage. Zopiclone was associated with minimal such effects. For bodily symptoms, zopiclone produced some headache, a metallic taste, and some blurring of vision; temazepam induced nausea, memory impairment and pins and needles. Withdrawal effects on bodily symptom ratings were inconsistent and not affected by tapering off the dose. In conclusion, the administration of zopiclone tends to be associated with some dysphoric effects, temazepam with sedation. Rebound effects are minimal with zopiclone and reducing the dosage gradually does not seem necessary.(ABSTRACT TRUNCATED AT 250 WORDS)