Using ³¹P magnetic resonance spectroscopy, creatine kinase (CK) reaction kinetics was assessed in the forearm flexor digitorum profundus muscle of healthy young (n = 11, age 34.7 ± 5 yr) and older (n = 20, age 73.5 ± 8 yr) subjects at rest, intermittent exercise at 20% maximum voluntary contraction (MVC), and 40% MVC. Exercise resulted in a significant increase in the average ratio of inorganic phosphate (P_i) to phosphocreatine (PCr) from resting values of 0.073 ± 0.031 (young) and 0.082 ± 0.037 (older) to 0.268 ± 0.140 (young, P < 0.01) and 0.452 ± 0.387 (older, P < 0.01) at 40% MVC. At 40% MVC, intracellular pH decreased significantly, from resting values of 7.08 ± 0.08 (young) and 7.08 ± 0.11 (older) to 6.84 ± 0.19 (young, P < 0.05) and to 6.75 ± 0.25 (older, P < 0.05). Average values of the pseudo-first-order reaction rate k_(PCrATP) at rest were 0.07 ± 0.04 s¹ in the young and 0.07 ± 0.03 s¹ in the older group. At both exercise levels, the reaction rate constant increased compared with the resting value, but only the difference between the resting value and the 20% MVC value, which showed an 86% higher reaction rate constant in both groups, reached statistical significance (P < 0.05). No difference in the reaction rate constant between the young and older groups was observed at either exercise level. As with k_(PCrATP), the average phosphorus flux through the CK reaction increased during exercise at 20% MVC (P < 0.05 in the older group) but decreased toward resting values at 40% MVC in both groups. The data in our study suggest that normal aging does not significantly affect the metabolic processes associated with the CK reaction.