The application of cell culture techniques to the study of epithelial transport function is illustrated by the renal-derived MDCK system. MDCK cells display a typical epithelial morphology with an asymmetric localisation of the (Na+-K+)-ATPase to the lateral interspace membranes. Transepithelial ion transport is observed (using cell monolayers grown on permeable millipore filters clamped into standard Ussing chambers) which is sensitive to hormonal stimulation by adrenaline, exogenous ATP, PGE1, vasopressin and aldosterone. The development of epithelial characteristics in culture is discussed.