In the circular muscle of the fundic part of the guinea-pig stomach, a small tonic contraction could be repeatedly produced by carbachol in Ca-free solution containing 2 mM EGTA. The carbachol-induced response was gradually increased during prolonged exposure to Ca-free solution for 50 h, whereas a short treatment with 0.1-0.2 mM Ca suppressed the subsequent carbachol response in Ca-free solution. The response was not essentially modified by increasing the external K+ concentration to 40 mM. Calmodulin antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W-7) and trifluoperazine selectively suppressed the carbachol response in the presence of Ca (0.05 mM) and the contraction induced by Ca (0.1 mM), but they had little effect on the response to carbachol in Ca-free solution at a concentration of less than 10 microM. A vasodilator agent, N-(2-guanidinoethyl)-5-isoquinoline sulphonamide (HA-1004), inhibited the carbachol response both in the presence and absence of Ca, as well as the Ca-induced contraction, to a similar extent, provided that the external Ca concentration was less than 0.1 mM. These results led us to propose that the contraction evoked by carbachol in the absence of external Ca is mediated by a process independent of the Ca-calmodulin system.