Nine mild male diabetic patients were studied by intravenous glucose tolerance tests with and without pretreatment with caffeine. The dose of caffeine was 90 mg given as 195 mg of caffeine citrate. The parameters measured were blood glucose and serum insulin. Caffeine given 2 hours previously did not affect the mean glucose disappearance rate of the group, however, those with the highest rates did show a reduced rate after caffeine. Insulin levels showed a decrease particularly during the first five minutes after glucose. This finding was present particularly in those patients who exhibited the larger first peak. It is possible that a relationship exists between the magnitude of the caffeine effect upon early insulin secretion and the glucose disappearance rate.