An equilibrated copolymer of mixed cyclosiloxanes represented by cyclic [(PhMeSiO) 2 (Me 2SiO) 2], (PMxMMy), produced deleterious effects on reproductive processes in both female and male animals. In mature female rats, PMxMMy caused interruption of the estrous cycle. The animals remained in metestrus or permanently in diestrus as long as treatment continued. When administered to pregnant rats before implantation, it was either lethal to the zygotes or led to failure of implantation. When pregnant rats were treated after implantation, embryotoxic effects were manifested. Necrotic material accumulated in the uterus, and abortion followed during the last third of pregnancy. Similar interference with the course of pregnancy was observed in rabbits. Administration of PMxMMy (220 mg/kg/day) at various stages of pregnancy in rats, followed by spontaneous delivery, revealed no effect on surviving male pups; dead fetuses were apparently normal. Mothers treated from day 16 on produced female pups with a urogenital structural abnormality, associated with urinary incontinence. Atropine treatment permitted the females to survive and reach maturity. Like the males, they developed normally in every respect, but the urinary incontinence persisted. The reason for the incontinence could not be demonstrated. The abnormality was reproduced in a second experiment. Cyclic monophenylheptamethylcyclotetrasiloxane (PM 1MM 3) was also administered to rats at various stages of pregnancy, in po doses of 220, 100, and 22 mg/kg/day. It revealed equally deleterious effects on pregnancy but did not elicit the urogenital malformation. Urinary incontinence was manifest only to a slight degree. When PMxMMy was administered po to mature male rats for as long as 25 wk, minimal inhibition of spermatogenesis was seen at 1000 mg/kg but not at 100 mg/kg. Decrease in testicular size and marked inhibition of spermatogenesis occurred in some, but not all, rats given 2000 mg/kg topically. The male rabbit was found to be much more susceptible, and arrest of spermatogenesis occurred in all animals treated po or topically. Fertility tests (mating with normal females) carried out on rats after 25 wk of po treatment (100 or 1000 mg/kg/daily 5 times per wk) suggested that the treatment may have produced some functional defect in the viability, motility, or maturity of the spermatozoa. The effect was not seen after 10 wk of treatment.