We measured in vivo forward flux of the creatine kinase reaction in rat forebrain in young (Y: 6 month, n = 13), mid-aged (M: 12 month, n = 7) and aged (O: 27 month, n = 10) animals using 31P magnetic resonance saturation transfer. Forward flux was reduced in the aged rats (Y: 0.42 +/- 0.08; M: 0.41 +/- 0.10; O: 0.31 +/- 0.03 s(-1) +/- SD; p = 0.008 O vs. Y). In vitro studies in a subset of the same rats showed a parallel decline in CK activity (Y: 2.16 +/- 0.40; M: 2.17 +/- 0.25; O: 1.56 +/- 0.06 IU +/- S.D.; p = 0.002 O vs. Y). The in vivo spectroscopic and in vitro biochemical measures were significantly correlated. Reduced creatine kinase activity could account for the observed decreased forward flux in aging brain. Intracellular pH, phosphocreatine/inorganic phosphate ratio, and phospocreatine/gamma-adenosine triphosphate ratio did not differ between groups. Forward flux may represent a better measure of brain energy function than relative phosphocreatine or adenosine triphosphate levels observable in vivo.