The transport of mirex and chlordecone (Kepone) across the placental during late gestation and through the milk during lactation was investigated in the rat. In the placental transport study, doses of 5 mg/kg were administrered on Day 15, 18 or 20 of gestation and animals were killed 4, 24, or 48 hr after treatment. Both compounds crossed the placental and were present in the fetus at all examination times. Maternal tissue levels exceeded fetal tissues. No effects of gestational age at time of treatment or of the fetus in the uterus were seen. In the lactation study, doses of 1 or 10 mg/kg/day were administered on Days 2-5 postpartum and pups were killed at intervals up to 12 days after treatment. The secretion of milk appeared to be a major route of elimination for both pesticides for nursing females, and the greater amount of mirex excreted via the milk as compared with chlordecone is in agreement with differences in their reported octanol-water partition coefficients. Initially, mirex entered the milk more rapidly than chlordecone. After cessation of treatment, mirex milk levels fell quickly, but chlordecone levels remained fairly constant. In the pups, mirex tissue levels paralleled milk levels; chlordecone levels, however, continued to increase in the tissues throughout the observation period.