(+)-S-2-amino-6-iodoacetamidohexanoic acid (AIHA), an irreversible inhibitor of the ornithindecarboxylase and extrahepatic arginase enzymatic activities with antineoplasic properties, was evaluated for antifertility activity in pregnant rats by oral administration at different periods of gestation. Our results showed that doses of 10 and 20 mg/kg of AIHA orally administered produced a contraceptive effect when it was administered from days 2 to 5, and 8 to 12 of gestation, respectively. The gestation time was slightly shortened when AIHA was applied from day 15 until labor. No sign of external malformations in fetuses was observed. On the other hand, AIHA did not affect the total length of oestrous cycle at the same dosage level used to interrupt pregnancy. In ovariectomized immature rats, neither changes in uterine weight, premature vaginal opening, or cornified cells were found. However, AIHA enhanced the estradiol-induced increase in uterine weights when both were concomitantly administered.