Peroxisome proliferator-activated receptor (PPAR)-α, a transcriptional activator, regulates genes of fatty acid (FA) metabolic enzymes. To study the contribution of PPAR-α to exercise training-induced improvement of FA metabolic capacity in the aged heart, we investigated whether PPAR-α signaling and expression of its target genes in the aged heart are affected by exercise training. We used hearts of sedentary young rat (4 mo old), sedentary aged rat (23 mo old), and swim-trained aged rat (23 mo old, training for 8 wk). The mRNA and protein expression of PPAR-α in the heart was significantly lower in the sedentary aged rats compared with the sedentary young rats and was significantly higher in the swim-trained aged rats compared with the sedentary aged rats. The activity of PPAR-α DNA binding to the transcriptional regulating region on the FA metabolic enzyme genes, the mRNA expression of 3-hydroxyacyl CoA dehydrogenase (HAD) and carnitine palmitoyl transferase-I, which are PPAR-α target genes, and the enzyme activity of HAD in the heart altered in association with changes of the myocardial PPAR-α mRNA and protein levels. These findings suggest that exercise training improves aging-induced downregulation in myocardial PPAR-α-mediated molecular system, thereby contributing to the improvement of the FA metabolic enzyme activity in the trained-aged hearts.