The effects of changes in extracellular pH (pH o ) on intracellular Ca 2 + concentration ([Ca 2 + ] i ) or intracellular pH (pH i ) were measured simultaneously with muscle tension in canine tracheal smooth muscle strips. [Ca 2 + ] i and pH i were measured using the fluorescent dyes fura-2 and BCECF, respectively. During high K + -induced contractions (24.2, 36.4 or 72.7 mM) at pH 7.4, pH o was changed to 7.8 or 7.0 with NaOH or HCl, respectively. Induced changes in pH i were equal to ∼ 50 % of the changes in pH o . Alkalinization significantly increased [Ca 2 + ] i and enhanced muscle contraction at all concentrations of K + but did not alter the relationship between muscle tension and [Ca 2 + ] i . Acidification significantly decreased [Ca 2 + ] i without changing muscle tone; hence, the muscle tension - [Ca 2 + ] i relationship was shifted to the left. These results suggest that changes in pH o can alter airway smooth muscle tone by changing [Ca 2 + ] i and pH i .