We measured the oxygen cost of breathing during sustained voluntary normocarbic hyperpnea in 12 subjects (4 endurance trainers, 4 strength trainers, and 4 controls) before and after a 5-wk training program limited to the ventilatory muscles (Leith, D. E., and M. E. Bradley. J. Appl. Physiol. 41: 508-516, 1976). "Steady-state" measurements of oxygen consumption were made at pulmonary ventilations ranging from 103 to 250 l/min. There were marked differences in the relationship between the metabolic cost of breathing and pulmonary ventilations between subjects. Spontaneously chosen respiratory frequencies ranged from 80 to 120 breaths/min and varied widely, even in a given subject, suggesting that the optima for frequency are broad or that optimization was imperfect. The subject group who performed endurance training increased by 19% the level of hyperpnea that they could sustain for 7--15 min, and increased their oxygen consumptions during this hyperpnea by an average of 67%. Following a 15-wk period of detraining, endurance trainers had lost 50% of their gains in the ventilations that they could sustain and in the accompanying oxygen consumptions. We conclude that ventilatory muscle endurance training can appreciably increase the aerobic endurance of the respiratory muscles.