1. Contractile responses of guinea-pig tracheal smooth muscle to KCl, histamine, acetylcholine and prostaglandins were reduced after the removal of extracellular Ca2+ from the bathing medium. 2. KCl-induced contractures were more dependent on extracellular Ca2+ than acetylcholine-induced responses, but less so than histamine-induced contractures. 3. Responses to high concentrations of contractile agents were less dependent on extracellular Ca2+ than those to low concentrations. 4. Repeated additions of histamine or acetylcholine in the absence of extracellular Ca2+ resulted in progressively smaller contractile responses. 5. Treatment of tracheal muscle with EGTA considerably inhibited drug-induced contractures and abolished the response to KCl. 6. It is concluded that contractile responses of guinea-pig tracheal smooth muscle involve both the influx Ca2+ from the extracellular space and release of Ca2+ from intracellular sites. The relative contributions of these two Ca2+ sources depend on both the contractile agent and its concentration. Intracellular Ca2+ stores may be depleted by treatment with EGTA, or by repeated drug additions in the absence of extracellular calcium.