The effects of chronic exposure to low levels of lead via maternal milk during the neonatal period in the rat were evaluated by means of several activity measures, spontaneous alternation in the Y-maze, and weight data. Animals reared at the highest concentrations of lead showed retarded body growth. Lead-treated rats were found to be more active in the Y-maze and in the tilt-box, but not in the running wheel. Lead-treated rats showed lowered spontaneous alternation and increased squealing; results were dependent upon the concentration of lead on which the animals were reared. Activity and alternation data provide suggestive evidence of lead-induced damage of the hippocampus, a structure which undergoes much of its development during the stage in which the lead was administered. Behavioral parallels found between lead poisoning and childhood hyperactivity were discussed with respect to the utility of chronic low level lead administration as an animal model of minimal brain dysfunction hyperactivity.