Peripheral nerve lesions of nude mice inoculated with Mycobacterium leprae were examined from 1 year to 1 year and 8 months post-inoculation. Leprosy bacilli and intracytoplasmic foamy structures were found in the perineurial cells, endothelial cells of capillaries, Schwann cells, and axons of the myelinated nerve fibers inside the lepromas of foot pads. In the proximal portion of the sciatic nerve, bacilli were found chiefly in the macrophages of the endoneurial space. Bacilli were not found in the perineurial cells of the sciatic nerves. Marked fibrosis, hypertrophy or degeneration of Schwann cells, demyelination, axonal damage or irreversible destruction of nerve architecture were not observed clearly. Therefore, it is thought that nude mice inoculated with M. leprae are suitable models for the study of the early peripheral nerve lesions of lepromatous leprosy.