The developmental stages of lead encephalopathy in the suckling rat were examined ultrastructurally using a model in which microvascular alterations induced by lead are not accompanied by neurological symptoms. The primary effect of lead was found to be upon capillary basement membrane and the intercellular junctions of the endothelium. Structural alterations in the basement membrane and intercellular junctions of capillaries in the cerebral cortex and cerebellum were accompanied by increased capillary permeability as evidenced by perivascular edema, extravasation of red blood cells and leakage of exogenously administered horseradish peroxidase through zonulae occludentes, intercellular junctions and capillary basement membranes into the adjacent parenchyma. Collagen was found to be a major constituent of basement membranes and intercellular junctions of capillaries in the cerebral cortex and cerebellum. Normal development of capillary basement membrane in the cerebellum during the early post-natal period is significantly later than in the cerebral cortex which appeared to render cerebellar capillaries more susceptible to the toxic effects of lead.