Fifty-eight nondiabetic subjects and fifty-nine diabetic patients were studied to determine the pattern of postprandial plasma insulin levels. In nondiabetic subjects, including both healthy and “medical” (ill) patients, plasma insulin levels after lunch were higher in obese than in nonobese subjects but did not depend upon sex or age. The maximal value occurred forty-five to sixty minutes after the beginning of the meal. When obese and nonobese subjects were analyzed separately, no significant difference in plasma insulin values was observed between nondiabetic subjects and hyperglycemic diabetic patients not receiving insulin. A delayed insulin response occurred in obese diabetics. Obese untreated patients had higher plasma insulin levels than nonobese untreated and obese treated persons. Subjects receiving long-term oral treatment and untreated, nonobese diabetics had no difference in mean plasma insulin levels. Portal vein sampling in four maturity-onset diabetic subjects revealed low and sluggish insulin secretion after food intake. Insulin-treated diabetic patients without insulin antibodies had significantly higher plasma insulin levels than nondiabetic subjects one and two hours after the beginning of the meal. Patients receiving short-acting insulin just before lunch had higher postprandial plasma insulin values and lower blood glucose levels than diabetic subjects taking long-acting or intermediate insulin.