The renal collecting duct is composed of principal and intercalated cells, which serve to control the extracellular fluid volume and solute composition of the body. Inwardly rectifying K⁺ channels play an important role in collecting duct function. The aim of the following study was to examine an inwardly rectifying K⁺ conductance in a cultured mouse collecting duct cell line (M8). K⁺ currents were examined using the whole cell patch clamp technique. Clamp potential was stepped from a holding value of –40 mV, to between +100 and -100 mV in 20 mV steps. The current sensitive to 5 mM Ba²⁺ was measured in the presence of 135 mM KCl or 130 mM NaCl plus 5 mM KCl in the bath (both at pH 7.4 with 2 mM CaCl₂). The pipette solution contained 145 mM KCl with no added Ca²⁺ plus 0.5 mM EGTA (pH 7.4). Measurements were taken initially (~50 mS) after changing the potential and at steady state (SS) (~350 mS). Point conductance was calculated at +100 (G_out) and –100 mV (G_in). All values are expressed as means ± SEM. Statistical significance was tested using Student’s unpaired t test and assumed at the 5% level. With high KCl in the bathing solution the initial Ba²⁺-sensitive G_out was 86.58 ± 32.37 μS/cm² and the initial G_in was 159.22 ± 49.92 μS/cm² (n=8). G_in was significantly greater than G_out identifying a weak inwardly rectifying K⁺ conductance. SS conductances were not significantly different to initial values. With high NaCl in the bathing Ringer solution the initial Ba²⁺-sensitive G_out was 140.60 ± 19.81 μS/cm² (n=48). At 350 ms Ba²⁺-sensitive G_out had fallen to –9.69 ± 4.50 μS/cm². The initial Ba²⁺-sensitive G_in was 53.51 ± 5.45 μS/cm² (n=48) and this increased at 350 ms to 64.73 ± 7.76 μS/cm². Time-dependent changes in G_out and G_in were not observed with total whole cell conductances. These data are consistent with time-dependent changes in Ba²⁺ sensitivity of the whole cell K⁺ conductance in the presence of extracellular Na⁺. One explanation is that is at positive potentials Ba²⁺ ions are repelled out of the channel pore. The absence of time-dependent changes in Ba²⁺ sensitivity in the presence of extracellular K⁺ could reflect the presence of a K⁺ lock-in site, which prevents Ba²⁺ from leaving the pore when K⁺ is bound (Spassova & Lu 1999; Vergara et al. 1999).