Aim To evaluate the changes of nitric oxide synthase-nitric oxide system activity in rat cardiac fibroblasts (CF) derived from spontaneously hypertensive rat (SHR) and Wistar rat with and without carvedilol. Methods CF were iso-lated by trypsin digestion method. Nitric acid reductase method and spectrophotometry were used to detect the NO contents and NOS activity respectively with or without carvedilol. Results Under basal condition (72 h), NO content (66.6 ± 3.5 μmol/ L)ancl NOS activity (16.7 ± 0.7 kU/L)of CF derived from SHR were siginificantly lower than those of Wistar rats (80. 8 ± 6.2 μmol/L, 29.1 ± 2. 1 kU/L) ( P< 0.01) . Carvedilol could significantly increase NOS activity and NO content of CF derived from two groups, which showed dose-dependent effects. Moreover, NO contents increased with the enhancement of NOS activity, and there was significant positive relevance between them ( r = 0.911, P< 0.01) . Conclusion The NOS-NO system of CFin SHR was abnormal, which showed that NOS activity and NO content of CF in SHR were lower. Carvedilol could significantly increase NOS activity and NO content of CF derived from two groups, but the effect on CF of SHR was much more powerful than that of Wistar rats. The effect may be an important mechanism to control blood pressure.