A five-minute exercise test is described in which the ventilatory response to a known amount of work is measured and the result expressed in terms of ventilatory requirement at a standard work level of 300 kg.M. per min. Experimental validation of the procedure is presented, together with data on normal standards and on the influence of such factors as age, sex, smoking, and athletics. The effect of psychologic factors on the results, especially at low work levels, is indicated. The value of the test as a simple, practical, and routine method of assessing cardio-respiratory function in patients with cardiac or pulmonary disease, or in subjects claiming compensation, is briefly outlined.