The frequency properties of arterial beds in organs were studied by temporarily ligating the renal, the gastric, the splenic or the superior mesenteric arteries of rats. Blood-pressure waves of the tail arteries were recorded before and during the ligations, and were analysed by Fourier's transformation. Their frequency spectra have been found to change profiles following specific patterns with the ligations of different arteries. The results were significant with regard to the frequency selectivities of the organic arterial beds. Such frequency properties can be clearly explained when the circulation system is viewed as an electrical circuit network in which the organic arterial beds work as filters.