In this study, the scaling of the oxidative metabolic enzymes citrate synthase (CS) and cytochrome c oxidase (COX) and the glycolytic enzyme pyruvate kinase (PK) in the red (‘slow-twitch’, oxidative) and the white (‘fast-twitch’, glycolytic) locomotor muscle of young (~2 to ~16 months of age) Pacific bluefin tuna (Thunnus orientalis) during the ontogeny of red muscle endothermy was investigated. On a mass-specific basis (units g^-1 muscle tissue), CS activity scaled negatively with body mass with scaling coefficients of -0.12 for red muscle and -0.21 for white muscle, whereas COX activity did not scale in either muscle type and PK activity scaled positively in white muscle, with a scaling coefficient of 0.09, but did not scale in red muscle. Thus mass-specific metabolic heat production potential either decreased or remained constant in the red muscle during the ontogeny of red muscle endothermy. In contrast, total red muscle mass, total red muscle CS activity and total red muscle COX activity all scaled positively with body mass with scaling coefficients of 0.90, 0.78 and 0.92, respectively, and each of these correlated positively with the magnitude of the red muscle thermal excess. Thus, increasing total, but not mass-specific, metabolic heat production capacity contributed to the increasing red muscle thermal excess with increasing body size in juvenile T. orientalis. Additionally, for CS and COX, transcript abundance was a poor predictor of enzyme activity. Thus, transcriptional regulation played a limited role in determining the differences between the two muscle types and the scaling relationships for these enzymes.