In the steady state of work on a bicycle ergometer repeated i.v. injections of (+) tubocurarine produced a decrease in handgrip strength. In spite of this the intensity of work was maintained constant for 20 to 30 min. During this time pulse rate and blood pressure increased slightly and irregularity but the ventilation increased, both absolutely and in relation to the oxygen uptake, by up to about 50 %. The cardiac output was uninfluenced by the curarization. By adding CO2 to the inspired air the alveolar PCO2 was maintained at the normal exercise level. It is therefore assumed that all the known humoral factors controlling respiration in exercise must have been normal. The greatly increased ventilation must consequently have been caused by some nervous factors. The origin of these factors, whether central or peripheral, is discussed on the background of earlier experimental findings. A tentative explanation, based on the assumption that bicycle-work is performed by the mediation of the gamma-loop, is suggested. According to this the“nervous factor” of respiratory regulation in exercise may be the feed-back to the reticular formation of afferent impulses from the muscle spindles.