The ability of different genotypic muscarinic acetylcholine receptors (mAChR) to inhibit the neural K+-current, IM, was assessed in clones of NG108-15 mouse neuroblastoma × rat glioma cells transfected with DNA for the genomic mAChRs ml –4 using tight-seal, whole-cell patch clamp recording. No significant inhibition of IM was seen in native (non-transfected) cells, or in m2 or m4 DNA-transfected cells at concentrations of acetylcholine up to 1 mM or muscarine up to 100 μM. Both acetylcholine and muscarine produced complete inhibition of IM in m3 DNA-transfected cells, but only partial (50–60%) inhibition in m1 DNA-transfected cells at maximally effective concentrations. This difference could not be explained by differences in mAChR number, as measured by radioligand binding and was not eliminated by adding GTP to the pipette. It is concluded that genotypic m3 receptors couple most effectively to IM and that this may explain previously reported instances of pirenzepine-resistant IM-inhibition.