The effects of a minor tranquilizer, zopiclone, on endocrine system in male rats were assessed to clarify the mechanisms whereby large doses of zopiclone may induce testicular damage. 1) Zopiclone had no significant effect on plasma and tissue levels of hormones of the hypophyseo-gonadal or -thyroid system in rats given 10, 100 and 250 mg/kg/day orally for 28 days. 2) Rats given zopiclone in doses of 10 and 100 mg/kg/day orally for 14 days had lowered serum levels of PGE2 and PGF2 alpha but the testicular PGF2 alpha concentration remained unchanged. Therefore, oral administration of high doses of zopiclone to rats has no evident influence on endocrine system related to male reproduction. And these findings support our previous paper that repeated dose of zopiclone cause no significant change in plasma or epididymal TS levels, nor to any change in lipid peroxidation in testicular tissues.