AbstractThe fetotoxic and teratogenic potential of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) for rhesus macaques (Macaca mulatta) was tested through oral administration to monkeys early in pregnancy. A single or divided dose, 1 μg of TCDD/kg of body weight, was followed by abortion in 13 of 16 pregnant monkeys treated between days 20 and 40 of gestation. One of four aborted at 0.2 μg/kg, and two of two at 5 μg/kg. None of the mothers given 0.2 μg/kg showed signs of toxicity. Eight of the monkeys aborting at 1 μg/kg showed clinical toxicity 44 to 111 days after aborting, and three died. Both given 5 μg/kg became toxic soon after abortion and died. No malformations except for two minor palatal abnormalities of questionable significance were found in the six fetuses that were not aborted at doses of 0.2 and 1.0 μg/kg. These results indicate (1) that TCDD is fetotoxic at doses that frequently have delayed toxicity to the mother, but that conclusions about teratogenicity cannot be drawn, and (2) that pregnant rhesus females are more sensitive to the toxic effects of TCDD than any species tested but th