Macroscopic and microscopic structure of the uterus at 48 h after treatment with different dosages of RU 486.

On day 8.5 of pregnancy, mice were treated with 3 different doses (0.3, 2, or 20 mg/kg) of RU 486. Uterine tissues were examined 48 h after the treatment. (A) Treatment with 0.3 mg/kg RU 486 caused termination of pregnancy in 60% of mice (3/5). The weight of the uterus increased, and the abortion was incomplete. (B) Treatment with 2 mg/kg RU 486 caused termination of pregnancy in 100% of cases, and the aborted horn completely regenerated. (C) Treatment with 20 mg/kg RU 486 caused pregnancy loss in 100% of mice and led to uterine dropsy (fluid retention: 3/5 mice [60%]). Therefore, the 2 mg/kg dosage was considered optimal for inducing abortion.