Intestinal tissue from cats was homogenized and fractionated by ultrafiltration to a molecular range of about 500‐10,000 and separated by gel chromatography. The fractions from the gel filtrations were tested with regard to their ability to inhibit Na,K‐ATPase as assayed with ρ‐nitrophenylphosphate. The fractions that inhibited Na,K‐ATPase were pooled and named pooled fraction 2. Pooled fraction 1 contained molecules larger, and pooled fraction 3 and 4 the molecules smaller, than pooled fraction 2. The four fractions were assayed for natriuretic ‘activity’ on anaesthetized rats. Administering pooled fraction 3 (no Na,K‐ATPase‐inhibiting activity) i.v. augmented renal excretion of sodium and water, whereas the other pooled fractions did not exhibit any consistent natriuretic effect. Cardiac ventricular tissue from cats was fractionated in the same manner as was done for the intestine. None of the cardiac fractions contained any natriuretic material. It is proposed that the cat small intestine contains a na