In nine normal humans we compared the effects on diaphragm fatigue of whole body exercise to exhaustion (86-93% of maximal O2 uptake for 13.2 +/- 2.0 min) to voluntary increases in the tidal integral of transdiaphragmatic pressure (integral of Pdi) while at rest at the same magnitude and frequency and for the same duration as those during exercise. After the endurance exercise, we found a consistent and significant fall (-26 +/- 2.9%, range -19.2 to -41.0%) in the Pdi response to supramaximal bilateral phrenic nerve stimulation at all stimulation frequencies (1, 10, and 20 Hz). Integral of Pdi.fB (where fB is breathing frequency) achieved during exercise averaged 509 +/- 81.0 cmH2O/min (range 304.0-957.0 cmH2O/min). At rest, voluntary production of integral of Pdi.fB, which was < 550-600 cmH2O/min (approximately 4 times the resting eupenic integral of Pdi.fB or 60-70% of Pdi capacity), did not result in significant diaphragmatic fatigue, whereas sustained voluntary production of integral of Pdi.fB in excess of these threshold values usually did result in significant fatigue. Thus, with few exceptions (5 of 23 tests) the ventilatory requirements of whole body endurance exercise demanded a level of integral of Pdi.fB that, by itself, was not fatiguing. The rested first dorsal interosseous muscle showed no fatigue in response to supramaximal ulnar nerve stimulation after whole body exercise. We postulate that the effects of locomotor muscle activity, such as competition for blood flow distribution and/or extracellular fluid acidosis, in conjunction with a contracting diaphragm account for most of the exercise-induced diaphragm fatigue.