The feasibility of a novel method for the noninvasive and local assessment of creatine (Cr) and phosphocreatine (PCr) dynamics in human skeletal muscle based on (13)C magnetic resonance (MR) spectroscopy is presented. A high dose of Cr, labeled at the guanidino C-4 position with (13)C 11\% enrichment, was administered orally to a human subject for 5 days. Using a surface coil, (13)C MR spectra of the lower leg were acquired on a 1.5T MR system at regular time intervals during and after Cr supplementation. An almost twofold increase in the intensities of the resolved PCr and Cr (13)C-4 signals was observed during this period. The slow decrease in these signals to normal values after supplementation reflects the slow daily turnover of Cr. The PCr/Cr ratio did not appear to change over the whole measurement period. During exercise of the leg, reversible changes in PCr and Cr signals were observed, reflecting conversion by the Cr kinase reaction.