The effects of natrahagin, a Chinese cobra venom proteinase on hemorrheology and plasma fibrinogen level in rat was investigated. The results showed that intravenous injection of natrahagin (0.025-0.1 mg/kg) dose-dependently decreased the whole blood viscosity and the plasma viscosity (P < 0.05 or P < 0.01) in rat, but caused no significant effect on hematocrit (P > 0.05). Natrahagin also decreased the plasma fibrinogen level (P < 0.05 or P < 0.01) of rat in a dose-dependent manner at these doses.