Carbohydrate metabolism and insulin secretion were investigated in 26 patients with Parkinson’s disease before and during L-dopa treatment. Oral glucose tolerance tests were performed on 13 patients treated with L-dopa alone and on 7 patients treated with L-dopa combined with Carbidopa. Intravenous glucose tolerance tests were performed on additional 6 patients treated with L-dopa alone. Results indicate that chronic L-dopa administration does not modify glucose metabolism. It was observed only a significant decrease of insulin secretion after oral glucose in the early phase (15th day) of treatment with L-dopa alone. This temporary effect may be related to the peripheral conversion of L-dopa to dopamine since insulin secretion during combined therapy was normal. The mechanism by which L-dopa transiently inhibits insulin release is not clear. Perhaps the oral administration of L-dopa alone causes an alteration in some gastrointestinal functions which are involved in the handling of oral glucos