This study examined the effect of high- (75 Hz, 1 min) and low- (5 Hz, 1.5 min) frequency stimulation on contractile and biochemical properties of the diaphragm. Tension was reduced to 21 ± 1 and 54 ± 2% (SE) of the initial value after high- and low-frequency stimulation, respectively. After 0, 0.25, 1, and 2 min of recovery from high-frequency stimulation, 5 Hz elicited more force (expressed as % of initial tension) than 75-Hz stimulation. Time 0 recovery values were 21 ± 1 and 78 ± 6% of the initial force for 75- and 5-Hz stimulation, respectively. By 1 min of recovery, force elicited by 5-Hz stimulation had returned to the prefatigue value. In contrast, force production with 75-Hz stimulation did not fully recover until 10-15 min. After fatigue produced by low-frequency stimulation, force production with 5-Hz stimulation was reduced to 54 ± 2% of the initial tension, a value significantly lower than the 71 ± 2% of initial force elicited by 75-Hz stimulation. Force production with 5-Hz stimulation increased rapidly in the first 15 s of recovery (54 ± 2% at 0 and 70 ± 2% at 15 s) and by 5 min was significantly greater than the force elicited by 75-Hz stimulation (100 ± 3 vs. 93 ± 1%). As before, force production at 75-Hz stimulation did not fully recover until 10-15 min. Both fatigue protocols produced a significant prolongation in isometric twitch contraction and one-half relaxation times. Creatine phosphate (CP) concentration was reduced and muscle lactate increased by both fatigue protocols. No significant differences in CP or lactate recovery patterns existed between the high- and low-frequency groups. We conclude that faster recovery in force production at 5 vs. 75 Hz was due to a fatigue-induced leftward shift in the force-frequency relationship. The high correlation between muscle pH and peak tension during recovery (r = 0.87, P < 0.001) suggests that fatigue produced by both high- and low-frequency stimulation may be partially mediated by deleterious effects of H+ on excitation-contraction coupling.