We hypothesized that increasing skeletal muscle total creatine (Cr) content through dietary Cr supplementation would result in slower muscle phosphocreatine concentration ([PCr]) kinetics, as assessed using (31)P magnetic resonance spectroscopy, following the onset and offset of both moderate-intensity (Mod) and heavy-intensity (Hvy) exercise. Seven healthy males (age 29 +/- 6 yr, mean +/- SD) completed a series of square-wave transitions to Mod and Hvy knee extensor exercise inside the bore of a 1.5-T superconducting magnet both before and after a 5-day period of Cr loading (4x 5 g/day of creatine monohydrate). Cr supplementation resulted in an approximately 8% increase in the resting muscle [PCr]-to-[ATP] ratio (4.66 +/- 0.27 vs. 5.04 +/- 0.22; P < 0.05), consistent with a significant increase in muscle total Cr content consequent to the intervention. The time constant for muscle [PCr] kinetics was increased following Cr loading for Mod exercise (control: 15 +/- 8 vs. Cr: 25 +/- 9 s; P < 0.05) and subsequent recovery (control: 14 +/- 8 vs. Cr: 27 +/- 8 s; P < 0.05) and for Hvy exercise (control: 54 +/- 18 vs. Cr: 72 +/- 30 s; P < 0.05), but not for subsequent recovery (control: 41 +/- 11 vs. Cr: 44 +/- 6 s). The magnitude of the increase in [PCr] following Cr loading was correlated (P < 0.05) with the extent of the slowing of the [PCr] kinetics for the moderate off-transient (r = 0.92) and the heavy on-transient (r = 0.71). These data demonstrate, for the first time in humans, that an increase in muscle [PCr] results in a slowing of [PCr] dynamics in exercise and subsequent recovery.