Observations were made on mast cells in various parts of the human peripheral nervous system obtained at autopsy or by biopsy. The specimens belonged to two major categories, viz. specimens without histological signs of abnormality and those with evidence of disease. The main findings were as follows: Mast cells were found in peripheral somatic nerves, spinal nerve roots, dorsal root ganglia, paravertebral and prevertebral autonomic ganglia, their communicants and adjacent branches. The cells could frequently be seen close to the nerve fibres and to small blood vessels. Mast cells accumulated in damaged parts of peripheral nerves following trauma, in long standing cases of diabetes mellitus, in some cases of metachromatic leucodystrophy and globoid cell leucodystrophy, and in the lesions of Recklinghausen's disease. Nerves from cases of Morton's disease did not show this mast cell response, nor could any appreciable number of mast cells be observed in the early stages of uremic and cancerous neuropathies.