In morbid obesity, there is an increased hindrance to breathing caused by the effects of the increased mass on the chest wall and abdomen; subjects with morbid obesity can maintain eucapnia by increasing inspiratory neuromuscular drive and/or by altering central breath timing. We studied 23 eucapnic, obese subjects (greater than 190% predicted ideal weight), 7 males and 16 females with a mean age of 36.6 +/- 9.2 yr and 18 healthy, normal male subjects. Total lung capacity, functional residual capacity, and total thoracic compliance were significantly (p less than 0.05) reduced in the obese subjects. At rest, minute ventilation was significantly increased because of an increase in respiratory frequency, which in turn was due to a significant decrease in the expiratory time (TE) per breath; the ratio of inspiratory to expiratory time (TI/TE) was thus significantly altered, indicating an alteration in central breath timing. Resting inspiratory neuromuscular drive (as represented by mouth occlusion pressure) was significantly increased in the obese subjects, but tidal volume was not significantly altered. There was an increased ventilatory responsiveness to hypoxia and relatively decreased ventilatory responsiveness to hypercapnia in the obese subjects. These results indicate that morbidly obese subjects maintain eucapnia primarily by an alteration in central breath timing. Although these subjects have decreased responsiveness to CO2, putting them at some risk of developing respiratory failure under conditions of hypercapnic/hypoxic stress, it is possible that this is counteracted by the increased responsiveness to hypoxia.