THE RESPIRATION OF FOUR SUBJECTS EXERCISING ON A BICYCLE ERGOMETER WAS STUDIED IN A STEADY STATE BOTH AT ATMOSPHERIC PRESSURE AND AT REDUCED PRESSURE AND COMPARED WITH RESPIRATION AT REST. IN CONTRAST TO THE OPINIONS OF SEVERAL EARLIER RESEARCHERS, NO GENERAL INCREASE IN MEAN CAPACITY WAS FOUND DURING WORK. VERZAR'S HYPOTHESIS OF A THIRD FORM OF REGULATION THUS APPEARS TO BE INCORRECT.