Plasma insulin responses to a 4-hour glucose tolerance (100 g) were studied in urbanized Black people. Persons of normal weight without diabetes (12) and obese persons without diabetes (18) were compared with obese diabetics (19). Fasting serum ketone levels were measured, and the plasma potassium, triglyceride and growth hormone responses during the glucose tolerance test were determined. Obese subjects without diabetes had a twofold greater total plasma insulin response (area under curve) than their counterparts of normal weight, but there was a progressive fall in total plasma insulin response from subjects with mild diabetes (with fasting normoglycaemia) to those with severe diabetes (with fasting hyperglycaemia). The early plasma insulin responses of the group with mild diabetes were significantly impaired, and the peak response was only reached at 120 minutes. The subjects with severe diabetes had a flat insulin response curve. Fasting serum ketone levels were highest in the group with severe diabetes. The growth hormone responses were similar in all the groups. Plasma potassium and tryglyceride levels fell less during the glucose tolerance test in the group with severe diabetes than in the other three groups. These data indicate that insulin secretion is reduced in obese Blacks with chemical evidence of diabetes and this reduction becomes severe in the symptomatic diabetic.