Halothane is a potent bronchodilator. The effects of halothane on isolated canine tracheal smooth muscle contraction and intracellular Ca(2+) increment induced by a high concentration of K+ were investigated to clarify how this anaesthetic decreases intracellular Ca2+ concentration ([Ca2+]i), an important second messenger. The tension of the muscle strips was measured using an isometric transducer, and [Ca2+]i was measured using a surface fluorescence spectroscopy. Exposure to a 72.7 mM K+ solution increased muscle tension and [Ca2+]i. Halothane (1,2,3 and 4% at the vaporiser) was introduced by bubbling in the presence of the 72.7 mM K+, and significantly decreased both this elevated muscle tension and the [Ca2+]i in a concentration-dependent manner. Similarly, slowly decreasing concentrations of K+ (48.5, 36.4, 24.2 and 18.2 mM) significantly decreased both of these variables. There was no significant difference between the slopes of the two regression lines of changes in muscle tension and changes in [Ca2+]i. From the evidence that tonic increase in [Ca2+]i by high concentrations of K+ is due to the influx of Ca2+ via L-type voltage channels, halothane may modify the L-type channels to decrease Ca2+ influx. In conclusion, halothane inhibits the high K(+)-induced canine airway smooth muscle contraction by decreasing [Ca2+]i. The decrease in [Ca2+]i by halothane might be due to inhibition of voltage-operated channels, especially the L-type channels.