When infant rhesus monkeys were exposed to lead via the addition of lead acetate (0.5-9 mg/kg body weight) to their formula or by the consumption of lead particles from lead-based surrogate mothers, they developed symptoms of lead intoxication within 6 weeks. Seizures, muscular tremors, and altered social interaction were the predominant changes. Visual impairment was also apparent in the more severely affected animals. In the animals showing obvious symptoms lead levels varied between 300 to 500 mug/100 ml of blood. Even in those animals having blood lead levels below 100 mug, hyperactivity and insomnia were observed. When the exposure to lead was eliminated, seizures subsided and visual impairment was reduced; however, the abnormal social interaction persisted. These animals also experienced a gradual decline in hematocrit and hemoglobin values during the period of examination. Liver and kidney biopsies obtained from these lead-exposed animals revealed characteristic intranuclear inclusions.When adolescent and adult monkeys were exposed to doses of lead acetate similar to those employed in the infant experiments, lead levels in excess of 200 mug/100 ml of blood were recorded. However, there were no obvious behavioral abnormalities observed. There were, however, numerous lead inclusion bodies in kidney biopsy specimens from these animals. These data suggest that, like man, the infant nonhuman primate is much more susceptible to lead intoxication than is the adult. The clinical and behavioral changes recorded in these infant rhesus monkeys suggest their use as an experimental model to evaluate lead intoxication.