This study was designed to evaluate the role of endogenous ouabain (EO) in the development of hypertension in 1k1c (one kidney, one clip) hypertensive rats. First, the EO content of the serum of 1k1 c hypertensive rats and normal Sprague-Dawley (SD) rats was detected by the enzyme linked immunosorbent assay method (ELISA). Second, blood pressure changes in the 1k1c rats were recorded directly after the 1k1c rats were injected randomly with anti-ouabain antibody, normal rabbit IgG, and normal saline, respectively. The results showed that EO levels in the serum of 1k1c hypertensive rats were significantly higher than those of normal SD rats (2.25±0.92g/l vs. 1.12±0.17μg/l, p<0.01), and correlated significantly with systolic blood pressure (r=0.59, p<0.05). Anti-ouabain antibody was able to significantly decrease the blood pressure of 1k1c hypertensive rats in a dose-dependent manner, while normal rabbit IgG or normal saline was not. These results indicate that endogenous ouabain might play an important role in the development of hypertension in 1k1c hypertensive rats. (Hypertens Res 2000; 23 Suppl: S61-S65)