A survey of the creatine and creatinine excretion of large numbers of mentally defective patients in circumstances which precluded both keeping them on creatine-free diet and the obtaining of 24-hour specimens, led Penrose and Pugh (1939) to the use of the creatine creatinine ratio in early morning specimens of urine as an index of the metabolism of these substances in each patient, as compared with controls. The use of the ratio was found to yield results of qualitative significance. Patients suffering from conditions known to affect creatine-creatinine metabolism, such as muscular dystrophies, diplegias, and hyperthyroidism, were readily picked out, and the creatinuria of children was marked; conclusions could be drawn as to the creatine-creatinine metabolism in certain other conditions. In the above survey the numbers allowed of statistical treatment to a considerable extent, and this confirmed the validity of the conclusions drawn from the ratio.