The circadian spontaneous locomotor activity of rats exposed to 0.1 mg lead/kg, po from 3 days until 4 and 6 weeks of age was similar to that of controls. However, hyperactivity during initial hours of recording was observed in rats that were treated with lead (Pb) until 8 weeks of age. When treatment was discontinued for 2 weeks, previously Pb-exposed rats had a tendency to be hypoactive. The elevated locomotor activity in 8-week-old lead-treated rats was not accompanied by any significant changes of noradrenaline levels in the cerebral cortex or hippocampus. Alterations in noradrenaline content of the hippocampus were, however, observed in rats that had been treated with Pb for 4 and 6 weeks. The turnover rate of noradrenaline in the hippocampus was also found to be significantly reduced following treatment for 6 weeks. Regional distribution of Pb in the brains of lead-exposed rats showed a large accumulation of the metal in the hippocampus. The alterations of the noradrenergic function in the hippocampus may be associated with the preferential storage of lead in this region.