The present study investigated the in vitro effects of extracellular ATP when applied to the basolateral border of renal epithelial cells in culture. We used the A6 epithelium cell line, derived from the distal renal tubules of the South-African clawed frog Xenopus laevis, as a model for the principal cells of the cortical collecting duct that exhibits both Na⁺ reabsorption and Cl^– secretion in response to specific agonists. Epithelial polarisation was facilitated on permeable Anopore filter supports (pore size 0.2 µm). We continuously monitored the changes in transepithelial conductance (G_T) and short-circuit current (I_sc). Data are represented as means ± S.E.M. Upon the addition of ATP (5 µM) to the basolateral border, I_sc rapidly increased in a biphasic manner with an initial rapid increase from 1.1 ± 0.2 to 3.0 ± 0.4 µA/cm² with a parallel rise in G_T from 0.13 ± 0.01 to 0.15 ± 0.01 mS/cm² (N=6). Both increases were transient and partly recovered within 3 min. A second rise was observed for both parameters, albeit with a much slower time course, reaching a maximum ca. 20 min after the addition of ATP for G_T at 0.16 ± 0.02 mS/cm² and for I_sc at 5.5 ± 0.5 µA/cm². In the presence of amiloride (50 µM), the second phase of the increases in G_T and I_sc was completely abolished, whereas the initial rapid response remained. This indicates that the second phase of the changes in G_T and I_sc reflect Na⁺ transport from the apical to the basolateral border. When substituting Cl^– for SO₄^2- in the basolateral bath, the first phase was overruled, whereas the second phase was still expressed. This is consistent with the process of Cl^– transport from the basolateral to the apical compartment during the first phase of the increases in G_T and I_sc. Our observations indicate that basolateral ATP elicits the immediate activation of Cl^– secretion followed by a much slower activation of Na⁺ reabsorption. In a previous study (1), we observed similar responses in transepithelial transport during hypotonicity, suggesting that ATP, released across the basolateral border of the epithelium in response to cell swelling, underlies the biphasic increases of I_sc and G_T in these conditions (2). Further investigations are required to identify the receptors and the signal transduction pathways involved.