The Madin-Darby canine kidney (MDCK) cell line, which exhibits properties indicative of a distal tubule origin, evidently binds and responds to mineralocorticoid hormones. We investigated the effects of aldosterone and deoxycorticosterone on protein synthesis in MDCK cells grown either in medium supplemented with serum or in a hormonally defined, serum-free medium. Aldosterone induced the synthesis of at least 2 membrane proteins with molecular weights of 35000 and 14000. The MDCK line may prove a useful model system for examining the mechanism of mineralocorticoid-regulated sodium transport and, in particular, the identification and study of hormone-induced proteins in a homogeneous cell population.