Seven patients suffering from maturity on-set diabetes mellitus were given orally 100 mg of 14C-labelled butylbiguanide, specific activity 1.40 or 1.23 muCi/mg, resp. Three days before oral administration, two of the patients had received an i.v. injection of 50 mg butylbiguanide labelled with 120 muCi 14C. The radioactivity in the blood of the patients was followed up during the first 12-h period after administration of the drug. For determination of the radioactivity in the urine aliquots of three 24-h portions were measured. Furthermore, the radioactivity was checked of each individual sample of faeces for the first 72 h after administration. The radioactivity in the exhaled air was also measured. By comparison of the excretion after i.v. and oral application an absorption efficiency of 90% to 92% was calculated. Butylbiguanide is almost exclusively and fast excreted via the kidney. 86.5% of the i.v. administered material was eliminated within 24 h and 88.1% within 3 d in the urine of a person without kidney disease. Elimination through faeces was negligible, 0.2% in a person without kidney disease and 0.7% in a patient with renal insufficiency. The data obtained from the exhaled air show that there is only a negligible break-down of butylbiguanide, if any, to CO2 in man.