Muscular ketone body metabolism was studied by the registration of arterial-deepvenous concentration differences of glycerol, beta-hydroxybutyrate and acetoacetate, and of muscular blood flow in 53 healthy volunteers and 23 juvenile diabetics. In six subjects of each group substrate metabolism was also investigated during the intrabrachial arterial infusion of metaproterenol (0.4 microgram/min). Identical rates of muscular glycerol release and ketone's fractional extraction were obtained in healthy and diabetic subjects during the basal period. A significant correlation between muscular ketone extraction and arterial ketone concentration was calculated in both groups. During the infusion of metaproterenol the diabetics, unlike the controls, revealed enhanced muscular glycerol output and reduced fractional extraction of beta-hydroxybutyrate. These findings are in line with the present view that the impairment of muscular ketone body utilisation in diabetic ketoacidosis may at least partly be due to enhanced muscular lipolysis.