In gross behavioral observation the minimum effective dose of zopiclone was intravenously 0.06-0.25 mg/kg and intragastrically 1-4 mg/kg in rhesus monkeys. Zopiclone suppressed barbital withdrawal signs of rhesus monkeys at a single dose of 16 mg/kg or more. The suppressing effect of zopiclone 32 mg/kg i.g. was comparable to diazepam 8 mg/kg. Crab-eating monkeys treated with zopiclone at doses up to 32 mg/kg twice daily for up to 8 weeks manifested intermediate-grade signs upon withdrawal (i.e., less severe than after treatment with diazepam and similar to nitrazepam). Zopiclone was self-administered by rhesus monkeys relatively frequently by the intravenous route; the infrequent intragastric rate was similar to or slightly higher than with diazepam.