With the aid of the halide-sensitive dye 6-methoxy-N-ethylquinolinium iodide (MEQ), changes in intracellular Cl^- concentration were measured to characterize the role of Ca²⁺-dependent Cl^- channels at the rat distal colon. In order to avoid indirect effects of secretagogues mediated by changes in the driving force for Cl^- exit (i.e., mediated by opening of Ca²⁺-dependent K⁺ channels), all experiments were performed under depolarized conditions, i.e., in the presence of high extracellular K⁺ concentrations. The Ca²⁺-dependent secretagogue carbachol induced a stilbene-sensitive Cl^- efflux, which was mimicked by the Ca²⁺ ionophore ionomycin. Surprisingly, the activation of Ca²⁺-dependent Cl^- efflux was resistant against blockers of classical Ca²⁺ signaling pathways such as phospholipase C, protein kinase C and calmodulin. Hence, alternative pathways must be involved in the signaling cascade. One possible signaling molecule seems to be nitric oxide (NO) as the NO donor sodium nitroprusside could induce Cl^- efflux. Vice versa, the NO synthase inhibitor N-ω-monomethyl-arginine (l-NMMA) reduced the carbachol-induced Cl^- efflux. This indicates that NO may be involved in part of the signaling cascade. In order to test the ability of the epithelium to produce NO, the expression of different isoforms of NO synthase was verified by immunohistochemistry. In addition, the cytoskeleton seems to play a role in the activation of Ca²⁺-dependent Cl^- channels. Inhibitors of microtubule association such as nocodazole and colchicine as well as jasplakinolide, a drug that enhances actin polymerization, inhibited the carbachol-induced Cl^- efflux. Consequently, the activation of apical Cl^- channels by muscarinic receptor stimulation differs in signal transduction from the classical phospholipase C/protein kinase C way.