Study of the in vitro behavior of strips of guinea pig bladder and taenia coli demonstrated that: Both bladder and taenia temporarily retain their ability to contract in Ca-free solutions, but the magnitude of this response decays with time. Carbachol is capable of producing contraction in Ca-free solution for a longer period of time than K depolarisation. Once lost, the ability of carbachol (but not K) to contract the tissues in Ca-free solution can be temporarily restored by a brief application of high Ca. The size of the carbachol contraction in Ca-free solution is reduced in Na-free solution, suggesting that membrane-bound Ca may not play a major role in this response. In depolarised bladder exposed to nifedipine 2 X 10(-7) M, carbachol can only elicit 1 large contraction, suggesting depletion of an intracellular source. It is concluded that whereas the response of the bladder to depolarisation depends primarily on extracellular Ca, the response to carbachol may also involve release of stored Ca and that the bladder, like other smooth muscles, appears to contain agonist-releasable intracellular Ca stores.