Aging induces complex changes in myocardium bioenergetic and contractile properties. Using F344BNF1rats, we examined age-dependent changes in myocardial bioenergetic enzymes (catalytic activities and transcript levels) and mRNA levels of putative transcriptional regulators of bioenergetic genes. Very old rats (35 months) showed a 22% increase in ventricular mass with no changes in DNA or RNA per gram. Age-dependent cardiac hypertrophy was accompanied by complex changes in mitochondrial enzymes. Enzymes of the Krebs cycle and electron transport system remained within 15% of the values measured in adult heart, significant decreases occurring in citrate synthase (10%) and aconitase (15%). Transcripts for these enzymes were largely unaffected by aging, although mRNA levels of putative transcriptional regulators of the enzymes (nuclear respiratory factor (NRF) 1 and 2 α subunit) increased by about 30%–50%. In contrast, enzymes of fatty acid oxidation exhibited a more diverse pattern, with a 50% decrease in β-hydroxyacyl-CoA dehydrogenase (HOAD) and no change in long-chain acyl-CoA dehydrogenase or carnitine palmitoyltransferase. Transcript levels for fatty acid oxidizing enzymes covaried with HOAD, which declined significantly by 30%. There were no significant changes in the relative transcript levels of regulators of genes for fatty acid oxidizing enzymes: peroxisome proliferator-activated receptor-α (PPARα), PPARβ, or PPARγ coactivator-1α (PGC-1α). There were no changes in the mRNA levels of Sirt1, a histone-modifying enzyme that interacts with PGC-1α. Collectively, these data suggest that aging causes complex changes in the enzymes of myocardial energy metabolism, triggered in part by NRF-independent pathways as well as post-transcriptional regulation.Key words: PGC-1a, fatty acid oxidation, nuclear respiratory factor (NRF), PPAR, coactivator, transcriptional regulation.