Administration of indomethacin (Id, 8 mg/kg twice daily), a prostaglandin (PG) synthetase inhibitor, on Days 4-7 of gestation resulted in a nearly complete inhibition of fetal development in rabbits. The small number of fetuses which survived to Day 14 were smaller than normal as were their associated decidual and placental tissues. In contrast to results with indomethacin in some other experimental animals, no evidence for teratologic effects was seen. Concomitant administration of estradiol-17β, progesterone, LH or LH/FSH with Id did not reverse the antifertility effects of the drug. PGE2 and PGF2α partially reversed the reduction in numbers of live fetuses caused by Id. PG's did not, however, reverse the decline in fetal, placental or decidual tissue weights. Id alone had no apparent effects on luteal weights.