From the dried body of Naja naja Kaouthia from which the internal organs had been removed, three hypotensive compounds were isolated and one of them was identified as inosine.Inosine produced a transient increase in blood pressure followed by a prolonged fall lasting for 30 min in spontaneously hypertensive rats. Next, the relationship between the chemical structure and the hypotensive effect was examined by studying ribose-modified derivatives of inosine. 2'-Deoxyinosine and 3'-deoxyinosine completely lacked hypotensive effect. Phosphorylation at the 3' or 5'-position except for inosine 5'-diphosphoribose and 5'-chloro-5'-deoxyinosine slightly decreased the hypotensive effect. The results indicate that the ribose configuration is important in relation to the hypotensive effect of inosine.