Effects of RU 486 (10 mg.kg-1, per.os) were assessed at mid-pregnancy in the rat. One hour after RU 486 treatment, myometrial electrical activity stayed low. It increased from the 3rd hour after administration of RU 486 and a perfect synchronization of the bursts of the action potentials was observed from the 6th h to the 24th h. Tissular steroid hormones and PGF2 alpha, evaluated at hour 6 after RU 486 administration, showed a decrease of progesterone concentrations in both myometrium and uterus. Estradiol levels decreased in uterus whereas, PGF2 alpha levels increased in both myometrium and uterus. These results show for the first time that RU 486 strongly increases the myometrial electrical activity in the rat at mid-pregnancy. This action was closely related to E2, P4 and PGF2 alpha concentrations.