K⁺ channels expressed in the renal collecting duct have a number of roles, including K⁺ secretion, maintaining the membrane potential and cell volume regulation (Giebisch & Wang, 1996). Previous studies have shown functional and molecular evidence for a number of different inwardly rectifying K⁺ channels (Kir), which could be involved in these roles. The aim of the following study was to investigate the presence of Kir channels in a cultured mouse collecting duct cell line (M8). K⁺ currents were examined in whole cell patches with either 135 mM KCl or 130 mM NaCl solutions in the bath. The pipette contained a 145 mM KCl solution, which contained EGTA and no added Ca²⁺. Clamp potential was stepped from a holding value of –40 mV, to between +100 and -100 mV in -20mV steps. In unpaired cells, the current sensitive to 5 mM barium (Ba²⁺) was determined. Whole cell currents were recorded either in the control circumstance or in the presence of 100 μM ATP or a non-hydrolysable ATP derivative, adenosine 5’-(β,γ-imido)triphosphate (AMP-PNP). On achieving the whole cell configuration currents increased to a peak, followed by a fall in magnitude to steady-state. The Ba²⁺-sensitive current (I_Ba) was measured at the peak current level and again after 5 minutes. All values are expressed as means ± SEM. Statistical significance was tested using Student’s paired and unpaired t tests and ANOVAs, and assumed at the 5% level. In paired cells with symmetrical KCl solutions the Ba²⁺ – sensitive conductance demonstrated inward rectification, outward and inward conductances were 94.01 ± 27.28 μS/cm² and 158.91 ± 35.47 μS/cm², respectively (n = 11). With high NaCl in the bathing solution peak I_Ba at +100 mV was 24.2 ± 3.25 pA/pF. This fell to 12.6 ± 2.23 pA/pF (n = 32) over 5 min. In unpaired cells initial I_Ba with ATP and AMP-PNP in the pipette was 22.7 ± 5.11 pA/pF (n = 13) and 20.6 ± 2.66 pA/pF (n = 14) respectively. There was no significant difference between the peak current levels. Rundown of I_Ba was prevented with ATP in the pipette, but not by AMP-PNP. The shift in I_Ba with ATP, 3.09 ± 2.65 pA/pF (n = 13), was significantly different to the decrease observed in the absence of pipette ATP and in the presence of AMP-PNP, 11.6 ± 1.80 pA/pF (n = 32) and 9.69 ± 2.09 pA/pF (n = 14), respectively. AMP-PNP was not significantly different to the control circumstance. These data indicate that cultured mouse collecting duct cells contain an inwardly rectifying K⁺ conductance. The K⁺ conductance is maintained by intracellular ATP, but not by AMP-PNP, suggesting that it is regulated via a mechanism dependent on ATP hydrolysis.