The possible functional coupling between β1-adrenoceptor and MaxiK channels which results in smooth muscle relaxation was examined in the guinea-pig esophageal muscularis mucosae. Isoprenaline-elicited relaxation of esophageal smooth muscle was confirmed to be mediated through β1-adrenoceptors as the response was competitively antagonized by a β1-selective antagonist atenolol with a pA2 value of 7.01. Iberiotoxin (IbTx, 10 -7 M), a selective MaxiK channel inhibitor, substantially diminished the relaxant response to isoprenaline. The extent of the MaxiK channel contribution to the relaxant response was 15-40% of the control response when estimated as the E50%-Emax responses to isoprenaline. The relaxation to isoprenaline was also attenuated by high-KCl (80 mM) to the same degree as the relaxant response generated in the presence of IbTx, and thus the estimated extent of the K + channel contribution was 10-40%. These findings indicate that β1-adrenoceptors are substantially coupled with MaxiK channels to produce relaxation of esophageal smooth muscle in the guinea-pig. Although MaxiK channels account for the contribution of K + channels to the β1-adrenoceptor-mediated relaxation in this smooth muscle preparation, their contribution seems to be less when compared to the β2-adrenoceptor-mediated relaxation of tracheal smooth muscle.