Sex-related differences in predisposition to heart diseases have long been recognized. The molecular and cellular bases for this difference are unknown. In this study we have compared expression of genes for various structural and functional proteins of muscle and interstitial compartments of the myocardium in the adult and neonatal, male and female rat heart. We have also compared cultured cardiac fibroblasts from male and female hearts with regards to gene expression and proliferative capacity. We showed that in the adult rats, the abundance of mRNAs for contractile proteins alpha- and beta-myosin heavy chain (MHC) is higher in the heart of female rats than in that of age-matched male rats. However, the difference in mRNA level for alpha-MHC was more drastic (736%, P < 0.001) than that for beta-MHC (469%, P < 0.001). mRNA levels for sarcomeric actin in the female heart were greater by 79% (P < 0.001). Collagen type I had a significantly higher (303%, P < 0.01) mRNA level in the female heart compared with the male heart. mRNAs for TGF-beta 1, cytoskeletal actin and connexin 43 were also higher (150%, P < 0.01; 130%, P < 0.01, and 150%, P < 0.01, respectively) in the female heart compared with age-matched male heart. There were no significant sex-related differences at the mRNA levels for the above proteins in ventricular tissue from neonatal male and female littermates. At the cellular level, cardiac fibroblasts obtained from adult and neonatal hearts of both sexes were comparable with respect to the abundance of mRNAs for collagen type I, TGF-beta 1 or cytoskeletal actin. However, DNA synthesis, as measured by [3H]thymidine incorporation, was higher (328%, P < 0.01) in cells from adult female heart compared with that in cells from adult male rat heart. This difference was even more pronounced in cardiac fibroblasts obtained from newborn female rats (933%, P < 0.001) compared with that in cells obtained from newborn male rat hearts. Together, these findings show that there are sex-related differences in gene expression for most major proteins in heart tissue and that this phenomenon is associated with the post-pubertal period. These findings further suggest that sex-related differential gene expression and DNA synthesis in cardiac cells are due to the regulatory effects of male- and female-specific hormones.