Methylguanidine (MG) has been demonstrated to be elevated in serum of uremic patients and its retention, per unit of serum creatinine concentration, to be higher in the anuric patients than in those with urine output exceeding 400 ml/day. The metabolic production of MG has been found to be increased in renal failure and correlated with the serum creatinine concentration, and its renal tubular excretion to be also elevated.In ten dogs submitted to ligature of the ureters, higher concentrations of MG were found in liver, muscle, and sciatic nerve as