We used the double sucrose gap apparatus to describe properties of electromechanical coupling in canine trachealis muscle, during infusion of elevated K+ and 5-hydroxytryptamine (5-HT). With 59 mM K+ and 5-HT, quasi-steady-state relationships of membrane depolarization and tension indicate electromechanical coupling, but for a given increase in tension much smaller membrane depolarizations were seen with 5-HT than with 59 mM K+. Evoked displacements of the membrane potential with anodal or cathodal current caused relaxations or contractions, respectively, and graded displacements resulted in graded tension changes. Reversal of 59 mM K+ membrane depolarization with long anodal current pulses resulted in nearly complete reversal of K+ tension, but reversal of 5-HT membrane depolarization caused tension reversal of only 20 to 40%. Data partially define electromechanical coupling in canine trachealis muscle provide evidence for membrane potential-independent tension with 5-HT at concentrations giving near threshold to maximal 5-HT tension.