Metabolic and endocrinologic effects of circulating maltose in diabetic states were studied. Six normal subjects, eight diabetic patients, and five mild ketoacidotic diabetic patients were given an intravenous infusion of 300 ml of 10% maltose over a 30-min period. The mean fasting blood glucose concentration in the ketoaci-dotic diabetic patients was 251±28 mg/100 ml, and this value was little affected by intravenous maltose administration, though an indefinite nadir of 242±18 mg/100 ml was attained at 120 min after the start of infusion. In contrast, maltose infusion produced a 53% increase in blood glucose over the basal level in the normal subjects. Maltose did not enhance insulin concentration of peripheral blood in the normal subjects despite such a significant increase in blood glucose. In diabetic and mild ketoacidotic diabetic patients, plasma insulin also remained unaltered after maltose infusion. In response to maltose, plasma FFA concentration was reduced by the following percentages at 30 min after infusion as compared with values in the basal state: -43% in normal subjects, -31% in diabetic patients, and -43% in mild ketoacidotic diabetic patients. Maltose infusion produced no change in blood acetone in diabetic patients. Similar metabolic effects of maltose were observed when 10% maltose solution was infused into alloxan diabetic rabbits at a rate of 0.2 ml/min over a 120-min period. These results indicate that intravenous maltose administration may be beneficial for the treatment of diabetes mellitus, since maltose is utilized by the patients in the insulin deficient state without any remarkable rise in blood glucose concentration.