AbstractThe ability of meprobamate, a commonly used tranquilizer, to be concentrated in human ovarian follicular fluid and to block superovulation in the mouse prompted an examination of its potential as a meiotic mutagen. Examination of in vitro meiotic behavior of mouse ova in a medium containing 1–10,000 μg/ml of meprobamate revealed that at 500 μg/ml abnormal second metaphase configurations were induced and that at a concentration of 750 μg/ml the progression of meiosis was essentially stopped. Superovulation was blocked in young mice each treated with 5 mg meprobamate and examination of intrafollicular oocytes principally revealed normal second metaphase configurations with first polar bodies. A single iv or chronic treatment throughout the time required for total maturation of a mouse follicle did not affect subsequent meiotic maturation of ova. The data suggest that the in vitro effects of the tranquilizer on mouse ova are prevented in vivo by an unknown barrier or mechanism within the ovarian follicle, and that at relatively high doses no detectable chromosome change in ova would be expe