The rat primary cultured-airway monolayer has been an excellent model for deciphering the ion channel after nystatin permeabilization of its basolateral or apical membrane. Inwardly rectifying K+ currents were characterized across the basolateral membrane in symmetrical HCO3--free high K+ Ringer's solution (125 mM) in this study. The potency of K+ channel inhibitors against K+ conductance was Ba2+ (IC50 = 5 muM) > Cs+ (IC50 = 2 mM) much greater than glybenclamide (IC50 > 5 mM) much greater than TEA (IC50 much greater than 100 mM). The application of basolateral Cs' changed K+ conductance into an oscillating current, and its frequency (holding voltage = -100 mV) increased with increase in concentration of basolateral Cs+ (0.05-5 mM) and in degree of hyperpolarization. Addition of basolateral Cs+ blocked inward current strongly at -100 mV and hardly at all at -60 mV, giving a sharp curvature to the I-V relation of the IRK current. RT-PCR, Western blotting, and immunohistochemical analyses showed that Kir2.1 might be present in basolateral membrane of tracheal epithelia and plasma membrane of pulmonary alveolar cells.