The effects of precoital intrauterine administration of Gossypol, Nonoxynol-9 or p-Aminobenzamidine on reproductive function were investigated in rats. The drugs were instilled in one uterine horn in the evening of proestrus prior to caging with a fertile male. The opposite side was similarly treated with the vehicle and served as control. Animals were killed in the morning following mating to assess the number of spermatozoa at the site of fertilization or on days 2-3 of pregnancy to determine the proportion of penetrated oocytes or on day 12 to count implanted embryos. Gossypol 0.05 to 5 mg or Nonoxynol-9 0.1 - 10 microliter reduced the number of spermatozoa reaching the ampullae and the number of penetrated oocytes in a dose-dependent fashion. At the highest dose both compounds prevented pregnancy exclusively in the treated side. p-Aminobenzamidine at a dose of 10 mg failed to affect the proportion of penetrated eggs found on days 2 or 3 of pregnancy. The results indicate that the rat bioassay is suitable to screen agents that can affect sperm function or viability since it can discriminate between drugs and doses and disclose differential effects upon sperm migration and fertilizing capacity.