Polychlorinated biphenyls are stable, lipophilic industrial compounds that are present in residue levels in human tissue, wildlife, and freshwater sediment. They are toxic, and are known to cross the placenta and intoxicate the fetus. Two large outbreaks of PCB poisoning have occurred in Asia; women pregnant at or after the exposures had children who were developmentally impaired. Laboratory experiments in rhesus monkeys and rodents, designed to assess neural or developmental effects, show altered activity levels, impaired learning, and delayed ontogeny of reflexes. Children exposed transplacentally to levels considered to be background in the U.S. have hypotonia and hyporeflexia at birth, delay in psychomotor development at 6 and 12 months, and poorer visual recognition memory at 7 months. Allowing for differences in testing, effects are roughly similar across species, but current methods used to calculate allowable or reference doses give results up to 4 orders of magnitude apart, with the lowest level based on the neurotoxicology level coming from the human data.