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Swelling-activated Cl- currents, I(Cl,swell), were measured during hyposmotic shock in white Leghornembryonic chick heart cells using the whole-cell recording of patch-clamp technique. Genistein, an inhibitorof protein tyrosine kinase (PTK), suppressed I(Cl,swell). Under isosmotic condition phorbol 12-myristate 13-acetate (PMA), an activator of PKC, elicited the Cl- current similar to that in hyposmotic solution, whereashyposmotic shock did not elicit I(Cl,swell) in chelerythrine chloride(an inhibitor of PKC)-treated cells. Con-focal microscopy experiments using FITC-phalloidin as a fluorescent label of F-actin showed that the actinnetwork was moved from cortical region of the cell to the center after hyposmotic shock as compared withthe image under isosmotic condition. When the cells were treated with cytochalasin B (CB) or cytochalasinD (CD) under isosmotic condition the disruption of the F-actin integrity was observed, and I(Cl,swell) wasnot elicited. With combination treatment of CB with PMA, hyposmotic solution could not elicited I(Cl,swell).The results suggested that the role of PTK, probably receptor tyrosine kinase, for regulation of I(Cl,swell)appeared to be at upstream site related to the role of F-actin. Then PKC signal pathway was activatedsomehow and finally change in the polymerization state of cytoskeleton led to activate the swelling-activatedCl- channels. These results demonstrate clearly that PTK, PKC and F-actin are important factors for reg-ulation of I(Cl,swell), in embryonic chick heart cells as compared with often controversial results reported indifferent cell types.