Aim To investigate the effect of aldosterone on expression of angiotensin Ⅱ type 1 (AT1) receptor in rat cardiac fibroblasts (CFs) and test the effect of aldosterone on AngⅡ induced collagen synthesis. Methods Cardiac fibroblasts culture were established from neonatal rats. Collagen synthesis was measured by 3 H proline incorporation after incubation with AngⅡ, aldosterone and AngⅡ plus aldosterone. The density of AT 1 recetpor was determined by 125 Ⅰ Ang Ⅱ saturation binding, and AT1 receptor mRNA levels were analyzed by quantitative reverse transcriptase polymerase chain reaction(RT PCR). Results AngⅡ increased collagen synthesis in cultured CFs; while aldosterone did not affect production of collagen. However, aldosterone could significantly potentiate the incremental effect of AngⅡ on collagen synthesis in CFs. The density of AT1 receptor inceased 2 fold and was accompanied by a 1.5 fold increase in the corresponding mRNA after treatment of aldosterone(10 -7 mol/L). Conclusion Aldosterone does not exert a direct effect on collagen synthesis in cultured rat CFs, but could influence the collagen output of CFs through the upregulated AT1 receptor.