We studied the functionally discrete calcium sources used by acetylcholine, 5-hydroxytryptamine, histamine and high K+ in the dog tracheal smooth muscle. The extracellular calcium dependence of their responses was assessed by altering the calcium and by pretreatment with the calcium antagonist, nifedipine. The intracellular calcium pool was assessed by studying the interactions between caffeine and the agonists in both skinned and unskinned preparations. The extent of overlap for the different calcium pools between the various agonists was determined by studying the dose–response relationships of these agents before and after pretreatment with another agonist, i.e., the conditioning agonist, in zero calcium conditions. The rank order of sensitivity to calcium removal and to nifedipine was histamine > KCl > 5-hydroxytryptamine > acetylcholine. Caffeine-induced atenuation of the agonist responses was predominantly through physiological antagonism. However, the caffeine responses in unskinned fibres were augmented by pretreatment with the agonists through both nifedipine-sensitive (as with KCl) and -insensitive (as with acetylcholine) mechanisms. The responses to acetylcholine and caffeine were inhibited by theophylline and forskolin. In the skinned muscle fibres, the pCa–tension relationship suggested high calcium sensitivity, a significant caffeine-sensitive calcium pool, and no evidence of calcium release by exogenous inositol trisphosphate. The results are consistent with multiple extracellular and intracellular calcium sources for the agonist responses. We observed considerable overlap of the calcium sources used by these agonists. Of the four agonists studied, histamine appeared to inhibit the release and sequestration of calcium utilized by the other agonists most effectively.