In three groups of normal subjects and in one group of patients with latent diabetes mellitus a study has been made of the effects of chlorpromazine (CPZ) on blood glucose and plasma insulin. CPZ 75 mg/day for 7 days did not alter the plasma insulin response after oral glucose; nor did CPZ 50 mg/day for 7 days affect the glucose assimilation rate or insulin response to glucose injection. Infusion of CPZ 50 mg in 60 min slightly increased the basal blood glucose level but had no significant effect on basal plasma insulin. The insulin/glucose ratio after the end of the infusion was significantly higher than during the period of infusion of the drug. In latent diabetic patients CPZ infusion significantly diminished the insulin/glucose ratio during an intravenous glucose tolerance test. These results suggest that, whereas prolonged treatment with low doses of CPZ did not modify glucose tolerance and glucose-stimulated pancreatic response, higher acute doses of the drug may induce hyperglycaemia and can inhibit insulin secretion both in normal man and in patients with latent diabetes mellitus.