Although the oral and parenteral administration of n-butylbiguanide in rabbits produces a hyperglycemic or hypoglycemic effect depending on the dose, the compound causes only a hypoglycemic response in normal as well as alloxan-diabetic rats (1). Clinical studies show a high incidence of gastrointestinal distress in patients as a remarkable side effect of the orally administered n-butylbiguanide (2). However, no pathohistological change in the gastrointestinal tract has been observed in rats after receiving the various doses of the compound for 6 months (1). Therefore, the anorexia and diarrhea of the diabetic patients treated with this compound are assumed to be functional in origin. Experimental studies on the distribution of hypoglycemic biguanides-¹⁴C by Kaneko (2), Wick et al. (3), Cohen et al. (4) and Beckmann (5) are consistent with each other in that biguanides distribute extensively to the gastrointestinal tract. The present experiments were designed to know the distribution of n-butylbiguanide-¹⁴C in tissues and the nature of urinary metabolites in mice.