Recent human isolated muscle fiber studies suggest that phosphocreatine (PCr) and creatine (Cr) concentrations play a role in the regulation of mitochondrial respiration rate. To determine if similar regulatory mechanisms are present in vivo, this study examined the relationship between skeletal muscle mitochondrial respiration rate and end exercise PCr, Cr, PCr/Cr, ADP and pH using ³¹P magnetic resonance spectroscopy (MRS) in 16 men and women (36.9±4.6 yr). The initial PCr resynthesis rate and time constant (T_c) were used as indicators of mitochondrial respiration after brief (10-12 s) and exhaustive (1-4 min) dynamic knee extension exercise performed in placebo and creatine supplemented conditions. The results show that the initial PCr resynthesis rate has a strong relationship with end exercise PCr, Cr, and PCr/Cr (r>0.80, P<0.001), a moderate relationship with end exercise ADP (r=0.77, P<0.001), and no relationship with end exercise pH (r=-0.14, P=0.34). The PCr T_c was not as strongly related to PCr, Cr, PCr/Cr and ADP (r<0.77, P<0.001 to 0.18) and was significantly influenced by end exercise pH (r=-0.43, P<0.01). These findings suggest that end exercise PCr and Cr should be taken in to consideration when using PCr recovery kinetics as an indicator of mitochondrial respiration and that the initial PCr resynthesis rate is a more reliable indicator of mitochondrial respiration compared to the PCr T_c.