Cortical thick ascending limbs containing macula densa plaques were dissected and perfused in vitro. Macula densa cell osmotic water permeability of the apical and basolateral membranes were measured by setting up osmotic steps across them in less than 0.1 s and following the ensuing time-dependent cell volume changes. The results of this study are in accordance with the view that the macula densa cells have a relatively low permeability to water. Apical and basolateral osmotic water permeabilities are 2.4 and 30.4 x 10(-4) cm3 s-1 osMolar-1 cm-2 basement membrane area, respectively. No infoldings were taken into consideration. These water permeabilities were not affected by maximal and supramaximal doses of vasopressin. This paper provides new insight into the physiological behaviour of this small, and almost inaccessible, sensing epithelial disc of cells which improves the understanding of its participation in the juxtaglomerular feedback response.