Sweat function was studied in patients with Parkinson's disease and in normal adults by sympathetic skin response, the bromphenol blue printing method and the silicone mould method. In patients with Parkinson's disease, dysfunction of sweating was classified into two types: one type involved the postganglionic fibres and the other involved the preganglionic fibres or the central nervous system. The latter was observed in patients with milder disease and the former was observed in patients with severe disease. The progressive involvement of sweat function in Parkinson's disease may reflect spread from the central nervous system or preganglionic fibres to postganglionic fibres. In a few patients the results of sweat tests were normal. Ceruletide increased sweating in Parkinson's disease patients, and decreased the prolonged latency of the sympathetic skin response. It is hypothesized that ceruletide facilitates the preserved somatosympathetic reflex of sweating.