Distinct inward-rectifier K+ channel subunits were expressed in Xenopus oocytes and tested for their sensitivity to the channel blocker quinidine. The 'strong' inward-rectifier K+ channel IRK1 was inhibited by quinidine with an EC5,, of 0.7 mM, while the 'weak' rectifier channel ROMKl was only moderately inhibited. ROMKI(N171D)-IRKlc_ter, chimeric channels, which carry both sites for strong rectification of IRK1 channels (the negatively charged D171 in the second transmembrane domain and the IRKl-C-terminus including E224), displayed strong rectiticatlon like IRK& but showed weak sensitivity to quinidine-like KOMKl, suggesting independence of quinidine binding and rectitication mechanisms. Moreover, BIRlO and BIRll, two strong rectifier subunits originally cloned from rat brain, exerted subunit-specific sensitivity to quinidine, being much higher for HIRll. Qulnldine blockade of IRK1 was not voltage-dependent, hut strongly dependent on the pH in the superfusate. These results *trongly suggest a subunit-specific interaction of inward-rectifier K+ channels with neutral quinidine within membrane lipid bilaytars.