Vasopresvin (AVP) causes electrogenic Cl secretion in mIMCD-K2 cells via cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is also an ATP channel and thus may increase extracellular ATP. In addition ATP is released from renal sympathetic nerves. The object of this study was to determine the effects of extracellular ATP on chloride conductance in mIMCD-K2 cells (gift from BA Stanton) Confluent monolayers (RT >1500 Ω cm2) on Transwell® membranes, were placed in an Ussing type chamber and short circuit current was measured. Application of ATP to either the apical or basolateral membrane led to rapid and transient increases in IscCl. The agonist profile ATP≡UTP>ADP indicates the presence of P2u purinergic receptors. UTP dose response showed the threshold dose lo be ∼300 nM, with no additional iespouse seen above 100 uM. Addition to the apical membrane elicited a greater response than addition to the basolateral membrane. Desensitization was rapid and complete, further additions of UTP/ATP 10 the same membrane resulted in no further stimulation. We believe this increased conductance is due to activation of outwardly regulated chloride channels. Apical application of UTP (or ATP, 100 uM) to cells previously stimulated with 1 nM AVP results in a non-additive UTP response superimposed on the AVP current, with current returning to pre-UTP levels. Addition of UTP basolaterally to AVP stimulaied cells leads to a non-additive response but with current returning to pre-AVP levels, i.e., abolished AVP stimulated current. These results imply differential signalling by P2u recptors according to membrane location.