A rapid sensitive GLC method for measuring urinary methylmalonic acid (MMA) employing boron trifluoride methylation of the acid is described. The method is sensitive to 1 mg MMA/L and affords MMA recoveries of 96-77 % over a concentration range of 5-2,000 mg MMA/L of urine. Mean daily MMA excretion in 10 normal subjects was 1.33 mg with a range of .56 to 3.2 mg. The daily excretion range of MMA in 5 vitamin B12subjects was 12.5-90 mg/day. These values compare with previously published levels determined using equally sensitive procedures.