Many different opinions are held about the value of metrazol (cardiazol) shock treatment. Large quantities of statistics are available which compare the recovery rates of treated and untreated patients (Bateman and Michael, 1940). Studies of the objective changes in treated cases have been less frequent. The present communication describes some experimental work directed towards elucidating clinical changes in patients treated with metrazol at the Ontario Hospital, London. Three psycho-physiological tests were employed: reaction time, tapping rate and strength of grip were measured. Psycho-physiological measurements, formerly used extensively (Kraepelin, 1920), have, for some years, been discredited as means for obtaining information about abnormal mental states (Stoddart, 1926). They retain their popularity as aids to the measurement of the effects of drugs such as alcohol and benzedrine (Thornton, Holck and Smith, 1939), and they have been used for the rating of general mental efficiency in aviators (Stamm, 1920), or as tests for loss of function due to anoxia (McFarland and Barach, 1937). Every test carries with it an element of experimental error and, in watching for improved performance attributable to a drug's effect, special care has to be taken to ascertain what improvement, if any, could be attributed to chance factors, to practice or to familiarity. Thus, the additional study of repeated performances of normal adults and control patients in the same test situation is necessary. It has, in the past, been amply demonstrated that the performances of psychotic patients on psychomotor tests are less efficient than those of normal people (Dorcas and Schaffer, 1934). Reaction time is lengthened, tapping rate slowed and strength of grip diminished. Moreover, an abnormal degree of variability in the response is common (Whipple, 1925) because the efficiency of some patients changes capriciously during a test. Other patients are found to show even less variety than normals (Shakow and Huston, 1936).