A depressor substance was found in acetone extract of bovine brain, greatly purified with the use of various chromatographic techniques, and tentatively named as D-I. It caused a sharp fall in arterial blood pressure of anesthetized cats with intravenous injections of 10 μg level of purified sample per kg animal body weight on a weight basis. Its hypotensive activity was not affected by previous treatment of cats with atropine, diphenhydramine, or hexamethonium. In the enzymic assay with the use of prostaglandin dehydrogenase (PGDH), neither prostaglandin E (PGE) nor prostaglandin F (PGF) compound was found. In the test of an incubation with a proteinase (nagase), the depressor activity was recovered almost completely. This depressor substance is definitely distinguishable from other depressor compounds and relatively resemble to lysolecithin, in the comparison of its attitude on TLC or in the course of purification process.