The involvement of functionally and molecularly defined basolateral potassium channels in the function of the distal nephron, as well as their implication in renal diseases, is emerging. By contrast, little is known about the molecular nature of basolateral potassium channels in proximal convoluted tubule (PCT) cells. Our goal is to identify the population of potassium channels located at the basolateral side of mouse PCT cells using the patch-clamp technique. PCTs were microdissected from collagenase-treated mouse kidney and transferred into a bath solution containing 140mM NaCl and 5mM KCl. In the presence of a high concentration of K+ (145 mM) in the patch pipette, two different channels types were oberved : a channel with inward (Gi) and outward (Go) conductances of 24.7 ± 1.5 pS (n = 16) and 4.2 ± 0.3 pS (n = 3), respectively, a channel with Gi and Go of 40.2 ± 0.6 pS (n = 12) pS and 4.4 pS (n = 2), respectively. All channels reversal potentials (25 pS channel : 40 ± 2 mV (n = 4) ; 40 pS channel : 39 mV (n = 2) were consistent with K+-selective channels. In cell-excised inside-out membrane patches, the 40 pS channel displayed internal Mg2+-dependent inward rectification (+ 1.2 mM Mg2+ : Go = 17.8 ± 5 pS (n = 3) ; 0 Mg2+ Go = 32.7 1.9 pS (n = 3)). This channel was also sensitive to internal pH (pHi) around the physiological range : as compared to channel activity at pHi 7.4, internal acidification to pHi 6.8 decreased channel NPo by 55 ± 22 %, while alkalinization to pHi 8 raised NPo by 70 ± 8 % (n = 3). RT-PCR data showed that the mRNAs of Kir4.2 and Kir5.1 subunits, but not of Kir4.1 subunit, were expressed in microdissected PCTs. Altogether, these results are consistent with 20 pS homomeric Kir4.2 and 40 pS heteromeric Kir4.2/Kir5.1 potassium channels in the basolateral membrane of mouse PCT and contrast with the well established presence of basolateral Kir4.1/Kir5.1 channels along the distal part of the nephron.