The metabolic, circulatory and respiratory adaptations to exercise in health have been briefly presented. A simple procedure is described which allows these adaptations to be conveniently studied in patients. Some examples are briefly presented to illustrate the information obtained from studies in which ventilation, O2 and CO2 exchange and the CO2 pressure in mixed venous blood and mixed expired gas are measured without blood sampling. A four quadrant diagram is used to illustrate the limits of adaptation of ventilation and cardiac output and the degree of anaerobic metabolism is estimated from the gas exchange data and the mixed venous Pco2.