In this study, K(+) channels present in the basolateral membrane of the distal convoluted tubule (DCT) were investigated using patch-clamp methods. In addition, Kir4.1, Kir4.2 and Kir5.1 inward rectifier channels were investigated using RT-PCR and immunohistochemistry (Kir4.1). DCTs were microdissected from collagenase-treated mouse kidneys. One type of K(+) channel was detected in about 50 % of cell-attached patches from the DCT basolateral membrane; this channel was inwardly rectifying and had an inward conductance (g(in)) of approximately 40 pS at an external [K(+)] of 145 mM. The current-voltage relationship was linear when inside-out patches were exposed to a Mg(2+)-free medium. Mg(2+) at a concentration of 1.2 mM considerably reduced the outward conductance (g(out)), yielding a g(in)/g(out) ratio of approximately 4.7. The polycation spermine (5 x 10(-7) M) reduced the open probability (P(o)) by 50 %. Channel activity was dependent upon the intracellular pH, with acid pH decreasing, and basic pH increasing, P(o). Internal ATP (2 mM) and Ca(2+) (up to 10(-3) M) had no effect. Channel activity declined irreversibly when the inner side of the patch was exposed to Mg(2+). Kir4.1, Kir4.2 and Kir5.1 mRNAs were all detected in the DCT. The Kir4.1 protein co-localised with the Na(+)-Cl(-) cotransporter, which is specific to the DCT, and was located on basolateral membranes. The DCT K(+) channel differs from other functionally identified renal K(+) channels with regard to its inhibition by spermine and insensitivity to internal ATP and Ca(2+). At the current state of knowledge, the channel is similar to Kir4.1-Kir5.1 and Kir4.2-Kir5.1 heteromeric channels, but not to Kir4.1 or Kir4.2 homomeric channels.