The relaxation rate of transdiaphragmatic pressure (Pdi) after voluntary contractions of the diaphragm slows with fatigue. We determined a range of values for the relaxation rate of mouth pressure (Pm) after voluntary contractions of the inspiratory muscles in 27 normal men and women at various lung volumes at rest. Values were similar for both sexes. The relaxation rates were similar at functional residual capacity (FRC) and below FRC, but were greater above FRC (p less than 0.05). In addition, we studied the effect of diaphragmatic fatigue induced by inspiratory resistive loading on the relaxation rates of Pdi and Pm with voluntary contractions of the inspiratory muscles in 6 subjects. With fatigue, the relaxation rates of Pdi and Pm both decreased by similar amounts, indicating that a decrease in the relaxation rate of Pm is as useful a predictor of inspiratory muscle fatigue as a decrease in the relaxation rate of Pdi has been shown to be for the diaphragm. The relaxation rate of Pm varies widely in normal subjects at rest, so isolated values do not indicate whether fatigue is present or developing. However, this may be predicted if relaxation rate decreases with serial measurements.