Noradrenaline and isoproterenol kinetics using intravenous infusion of L-3H-NA and of 3H-isoproterenol were investigated in eight Type 1 (insulin-dependent) diabetic patients without neuropathy and in eight Type 1 diabetic patients with autonomic neuropathy matched for age, sex and duration of diabetes. Resting plasma noradrenaline and adrenaline concentrations were reduced in patients with autonomic failure (p less than 0.05). The metabolic clearance rate of noradrenaline was similar in both groups of patients, and the appearance rate of noradrenaline in plasma was reduced in patients with autonomic failure (p less than 0.01). The disappearance of L-3H-noradrenaline from plasma after the infusion of L-3H-noradrenaline had been stopped was not different in patients with and without neuropathy. The metabolic clearance of isoproterenol was not influenced by the presence of autonomic failure and mean values were similar to the corresponding values for noradrenaline. Isoproterenol was only taken up by a non-neuronal uptake; this finding may indicate that neuronal uptake is not important for the inactivation of circulating catecholamines. Alternatively, because the non-neuronal uptake of isoproterenol is probably greater than that of noradrenaline, we cannot exclude the possibility that a small decrease in the neuronal uptake of noradrenaline was compensated for by a slightly higher non-neuronal uptake.