Vectorial transport of salt and water in the Madin-Darby canine kidney (MDCK) cell line is indicated by the formation of domes when a monolayer is grown on an impermeable support. We investigated aldosterone-induced dome formation and evaluated the dome as an experimental model. Transepithelial dome resistance was about 80 omega cm2 and constant when dome size exceeded 2.10(-4) cm2. The relative ion conductances (expressed as transference numbers) across the dome epithelium were tNa:tCl:tk = 0.64:0.24:0.06. They reflect the permeability properties of the paracellular shunt pathway tested at physiological concentrations of the individual ions. Aldosterone accelerated dome formation in serum-deprived MDCK monolayers. Prostaglandin E1 and transferrin were supportive but not essential for aldosterone-induced dome formation. After 72 h dome density was equal in monolayers cultured in serum-supplemented medium either in the presence or absence of mineralocorticoids. We conclude that aldosterone induces cell polarization in MDCK monolayers, leading to the formation of domes. The dome epithelium appears to be electrically isolated from the adjacent monolayer and can be studied by microelectrode techniques.