The present study was designed to determine if l-dopa alters endocrine function in humans. Patients with Parkinson's disease underwent control studies that included determination of basal 24 hr urinary 17-hydroxy and keto steroids and their response to metyrapone, urinary gonadotropins, serum thyroxine, 24 hr 131I uptake by thyroid gland and the plasma glucose, insulin and growth hormone responses to oral glucose, iv glucose, iv tolbutamide and iv insulin. During the third week of l-dopa therapy (6.0 g/day) the studies were repeated. l-dopa had no effect on thyroid or adrenal function. Metyrapone response and urinary excretion of gonadotropins were unaltered. Glucose decay constants and insulin secretion following the iv glucose and tolbutamide were not changed. The oral glucose tolerance was modified in that early insulin secretion (0 to 120 min) was significantly inhibited (60.6% of control study) and late plasma glucose (3 to 5 hr) was elevated. Plasma growth hormone was elevated following l-dopa and was not suppressed by either oral or iv glucose. The studies indicate that l-dopa causes an alteration in the response to oral glucose, and a stimulation of growth hormone secretion. The control studies in the Parkinsonian patients indicated that impaired growth hormone release from insulin-induced hypoglycemia (44% of patients) and abnormal iv glucose tolerance (50% of patients) may be associated with the disease.