The expression of genes encoding fatty acid utilization enzymes is coordinately downregulated during the development of cardiac hypertrophy and failure. However, molecular mechanisms that mediate this downregulation are unknown. Peroxisome proliferator-activated receptor (PPAR) response elements (PPREs) have been identified in promoters of many genes involved in fatty acid utilization, where they function as positive regulatory elements. PPARs bind to PPREs as heterodimers with retinoid X receptors (RXRs). Primary cardiac myocytes from neonatal rats were transfected with a reporter construct driven by the C promoter of rat acyl-coenzyme A synthetase (ACS) gene. Stimulation with phenylephrine, a potent inducer of hypertrophy, markedly downregulated the activity of this promoter. By use of electrophoretic mobility-shift assays (EMSAs) using PPRE in the rat ACS promoter as a probe, we found a sequence-specific protein–DNA complex in the nuclear extract from adult rat left ventricular (LV) myocardium. Supershift experiments revealed that this complex was immunoreactive for PPARα and RXRα. We compared the activity of this complex in LV nuclear extracts from Dahl salt-sensitive rats (DSs) with hypertension and control age-matched Dahl salt-resistant rats (DRs). Even at the stage of concentric LV hypertrophy with normal systolic function, the activity of the band was markedly diminished in DSs compared with DRs. However, immunoblot analyses showed no difference in LV expression levels of PPARα or RXRα between DSs and DRs. These findings indicate that a nuclear complex of PPARα/RXRα is present in adult rat LV and is markedly downregulated in the hypertrophied LV from DS rats, which may account for the loss of transcriptional activation. The downregulation of this complex precedes LV systolic dysfunction and is mediated at the posttranslational levels.