Isolated strips of canine tracheal smooth muscle rapidly lost their responsiveness to histamine when placed in a zero calcium Krebs buffer. Responsiveness to acetylcholine, however, was not rapidly lost, and following 120 min of incubation in zero calcium buffer with frequent washes, 10% of the contractile response still remained. The kinetics of each loss of response suggest that primarily a loosely bound source of calcium is mobilized by histamine and a more tightly bound source is mobilized by acetylcholine. Consistent with these data were the effects of the calcium antagonist verapamil. In normal calcium Krebs solution, dose-response curves to histamine were markedly reduced by verapamil while acetylcholine responses were relatively unaffected. In calcium depleted tracheal strips, indomethacin potentiated the calcium dose-response curve, determined by incremental readdition of calcium in the presence of histamine (10(-4) M), with comparatively little effect on the calcium dose-response curve in the presence of acetylcholine (10(-6) M). Also, in indomethacin pretreated tracheal strips, a reduction in the histamine-calcium dose-response curve could be produced by exogenous addition of 2.8 X 10(-9) M and 2.8 X 10(-8) M PGE2. In the acetylcholine-calcium responses there was a significant reduction only at 2.8 X 10(-8) M PGE2. These data suggest that histamine mobilizes primarily a loosely bound, possibly extracellular source of calcium necessary for contraction, and this histamine-stimulated calcium mobilization is sensitive to the effects of PGE2.