This paper briefly reviews our current effort to study the Ca2+ mobilization mechanism in enzymatically dispersed single smooth muscle cells. Each single cell obtained from guinea pig taenia caeci possesses two types of Ca2+ stores, one (S alpha) with both Ca(2+)-induced and IP3-induced Ca2+ release mechanisms and the other (S beta) with only IP3-induced Ca2+ release mechanism. After depletion of S alpha either with ryanodine treatment or with caffeine pretreatment, carbachol failed to induce Ca2+ release, while intracellular application of IP3 did induce Ca2+ release. Our results suggest that the difference between the agonist- and IP3-induced responses can be resolved by obligatory involvement of positive feedback control of IP3-induced Ca2+ release in the agonist-induced Ca2+ release. Furthermore, we were able to demonstrate that the dose-response relation in single cells shows an all-or-none feature, which seems at least partly due to the feedback control of Ca2+ release. We discuss the reasons why graded dose-response relation is obtained in bundles of smooth muscles, while the response of single cells is an all-or-none type.