Cardiac β-myosin heavy chain (β-MHC) gene expression is mainly regulated through transcriptional processes. Although these results are based primarily on in vitro cell culture models, relatively little information is available concerning the interaction of key regulatory factors thought to modulate MHC expression in the intact rodent heart. Using a direct gene transfer approach, we studied the in vivo transcriptional activity of different-length β-MHC promoter fragments in normal control and in altered thyroid states. The test β-MHC promoter was fused to a firefly luciferase reporter gene, whereas the control α-MHC promoter was fused to the Renilla luciferase reporter gene and was used to account for variations in transfection efficiency. Absolute reporter gene activities showed that β- and α-MHC genes were individually and reciprocally regulated by thyroid hormone. The β-to-α ratios of reporter gene expression demonstrated an almost threefold larger β-MHC gene expression in the longest than in the shorter promoter fragments in normal control animals, implying the existence of an upstream enhancer. A mutation in the putative thyroid response element of the −408-bp β-MHC promoter construct caused transcriptional activity to drop to null. When studied in the −3,500-bp β-MHC promoter, construct activity was reduced (∼100-fold) while thyroid hormone responsiveness was retained. These findings suggest that, even though the bulk of the thyroid hormone responsiveness of the gene is contained within the first 215 bp of the β-MHC promoter sequence, the exact mechanism of triiodothyronine (T₃) action remains to be elucidated.