After administration of a single dose (by mouth) of a series of symmetrical dialkylnitrosamines (nitrosodimethylamine — NDMA, nitrosodiethylamine — NDEA, and nitrosodibutylamine — NDBA) to rats in the maximal doses tolerated by pregnant animals (NDMA, 30 mg/kg; NDEA 200 mg/kg; NDBA 1200 mg/kg), only an increase in the mortality among the embryos was observed on the 3rd, 9th, 10th, and 12th days after fertilization. No teratogenic effect likewise was observed after the intraplacental injection of NDMA and NDEA (100–300 μg into each embryonic sac) on the 13th day of pregnancy. In experiments in which the compounds were given on the 21st day of pregnancy a transplacental oncogenic effect was found in the progeny: NDMA induced tumors in 5 of 20 (25%) animals and NDEA in 15 of 31 (48%) rats that survived until the time of appearance of the first tumor. Malignant tumors were frequently situated in the kidneys.