The teratogenic potential of the leukotriene antagonist LY171883, a novel antiasthma agent, was investigated in CD rats and Dutch Belted rabbits. Mated female rats were dosed with 0, 10, 65, or 425 mg/kg/day on gestation days 6 through 15 and killed on gestation day 20. Mated female rabbits were dosed with 0, 20, 65, or 200 mg/kg/day on gestation days 6 through 18 and killed on gestation day 28. Maternal toxicity was indicated at 425 mg/kg in rats and 200 mg/kg in rabbits by depressed body weight gain and food consumption. In the rabbit study four abortions occurred at 200 mg/kg, most likely secondarily to maternal toxicity. LY171883 did not cause embryo/fetal toxicity or teratogenicity in rats or rabbits at doses up to and including those that were maternally toxic.