Regulatory volume decrease and exocrine secretion studies suggest a functional relationship between K+ and organic anion efflux. To test the hypothesis that the expression of K+ channels and MRP1 is reciprocally related, we employed the patch clamp and RT-PCR techniques on weakly (H69) and strongly MRP1-expressing (H69AR) small cell lung cancer cells. H69AR cells do not express the time- and voltage-dependent delayed rectifying K+ current (Kv) reported earlier in H69 cells and confirmed here. About 80% of the Kv current in H69 cells inactivated at 0 mV, allowing us to identify other K+ currents present in these cells. Whole-cell currents from cells dialyzed and bathed in K-gluconate as the major ions exhibited inward rectification in both cell types. Inwardly rectifying (Kir) currents in both H69 and H69AR cells showed time-dependent activation and slow inactivation at large negative potentials. H69 cells also express a threefold larger Ca2+-stimulated K+-selective and iberiotoxin-sensitive current relative to H69AR cells. In excised inside-out patches exposed to 145 mM symmetrical K+ solutions, H69 cells expressed a voltage- and Ca2+-sensitive large conductance (128 ± 5 pS) K+ channel (MaxiK). MaxiK-like currents were not observed at the whole-cell or single-channel level in H69AR cells. RT-PCR identified MaxiKα transcripts in H69 but not H69AR cells. These results indicate that two K+ currents (MaxiK and Kv) and the organic anion transporter MRP1 are reciprocally expressed in H69 and H69AR cells. J. Cell. Physiol. 209: 535–541, 2006. © 2006 Wiley-Liss, Inc.