AbstractThe effects of acetylsalicylic acid, meclizine hydrochloride, methamphetamine hydrochloride, clomiphene citrate, vincristine sulfate, and 6‐aminonicotinamide on fetal development of the Macaca mulatta were studied.Two monkeys administered acetylsalicylic acid 40 mg/kg/day from the 25th day of gestation to term produced normal young. Seven pregnant monkeys given meclizine hydrochloride 10 mg/kg/day during various five‐day intervals had normal progeny. Five monkeys given methamphetamine hydrochloride 0.5 mg/kg/day from implantation to term had four normal young and one abortion. Clomiphene citrate administered to 18 pregnant monkeys during various stages of organogenesis resulted in 16 normal offspring and two stillbirths.Two of the five monkeys that received vincristine sulfate produced offspring with congenital malformations. One, given a single injection of 0.175 mg/kg on the twenty‐ninth day of gestation, had an offspring with encephalocele. One administration of 0.15 mg/kg on the twenty‐seventh day of gestation produced syndactyly in the offspring of another. The other monkeys, which received 0.15 or 0.20 mg/kg on the twenty‐eighth, thirty‐third, or thirty‐fourth day of gestation, produced normal offspring.Seven monkeys were treated with 6‐aminonicotinamide at different stages of gestation. Four, treated from the twenty‐sixth to twenty‐ninth days, aborted, the other three had normal young.Retrospective studies of therapeutic agents used at various times during pregnancy in monkeys in the colony have shown that hydroxyprogesterone caproate (Delalutin), chloramphenicol succinate (Chloromycetin), lincomycin hydrochloride (Lincocin), and benzathine penicillin G and procaine penicillin G (Longicil) have no apparent effect on the developing monkey fetus.