Dehydroepiandrosterone (DHEA) is a type of adrenal steroid. The concentrations of DHEA and its sulfate (DHEA-S) in serum reach a peak between the ages of 25 and 30 years and thereafter decline steadily. It was reported that DHEA-S concentration in humans is inversely related to death from cardiovascular diseases. In this study, we examined the effects of DHEA on regulation of collagen mRNA and collagen synthesis in cultured cardiac fibroblasts. Treatment with DHEA (10^-6M) resulted in a significant decrease in procollagen type I mRNA expression compared with controls. This was accompanied by a significant decrease in procollagen type I protein accumulation in the medium, and also a significant decrease in procollagen type I protein synthesis in the cellular matrix. Furthermore in order to confirm in vitro results, we administered DHEA to Sprague-Dawley rats, which were treated with angiotensin II for eight weeks to induce cardiac damage. Procollagen type I mRNA expression was significantly decreased and cardiac fibrosis was significantly inhibited in DHEA treated rat hearts without lowering the systolic blood pressure. These results strongly indicate that the DHEA can directly attenuate collagen type I synthesis at the transcriptional level in vivo and in vitro in cardiac fibroblasts.