The effects of treatment of rats with biphenylylbutyric acid were assessed by measuring the amount of acetate-1-C14incorporated into the cholesterol and phospholipid of several tissues. The drug inhibited the inclusion of the label into the plasma and hepatic lipid fractions. The treatment had no significant effect upon the lipid metabolism of either the testis or the adrenal. In the aorta, although the incorporation of the labeled acetate into cholesterol was unchanged, the amount of label found in the phospholipids was strikingly enhanced by the treatment. The unique nature of aortic lipid metabolism as evidenced by this work and previous reports is discussed.