The purpose of this review is to provide a selective, but critical, assessment of important findings derived from both epidemiological and laboratory studies suggesting that: (1) exposure to polychlorinated biphenyls (PCBs) and related halogenated aromatic hydrocarbons induces significant neurological and behavioral dysfunctions in humans and laboratory animals, particularly following exposure during gestation and lactation; (2) the neurochemical actions of PCBs depend on their structure and the developmental status of the animal at the time of exposure; and (3) the mechanisms responsible for these changes may involve alterations in basic cellular signaling processes and endocrine function that influence the synthesis and activity of important central nervous system neurotransmitters, the organization of the developing brain, and the behavioral responses to these environmental contaminants.