Enteroglucagon concentration in peripheral blood was determined before and after a test meal in 24 morbidly obese patients. Eighteen had jejunoileal bypass, 6 with a 3:1 and 12 with a 1:3 jejunoileal ratio of the functioning segment, and 6 were unoperated. All three groups exhibited an increment of enteroglucagon concentration after the meal. Both the fasting values and the postprandial integrated increments were higher in operated patients than in unoperated patients and higher after 1:3 bypass than after 3:1 bypass. The findings agree with the hypothesis that enteroglucagon secretion is stimulated by exposure of the lower bowel to upper-bowel content, and that the effect of enteroglucagon is, as seen after bypass operation, stimulation of growth and reduction of motility of the intestine.