The urinary clearance of pyridostigmine was studied in six patients with myasthenia gravis. In three patients on pyridostigmine alone, renal clearance ranged from 349 to 481 ml/min, corresponding to a pyridostigmine:creatinine clearance ratio of 2.64 to 3.46. In a patient on bendrofluazide as well as pyridostigmine, a similar clearance ratio was observed. By contrast, the urinary clearance of pyridostigmine and the pyridostigmine creatinine clearance ratio was reduced in two myasthenic patients concurrently treated with other basic drugs. It is suggested that these results may reflect competition for renal tubular excretion.