The cell-volume regulation by swelling-activated Cl⁻ current (I_Cl,swell) was studied in guinea pig ventricular myocytes, using a microscopic video-image analysis. We have previously shown that in ventricular cells depolarized in high-K⁺ ([K⁺]_o >45 mM) solution, an activation of the cyclic AMP-dependent Cl⁻ current (I_Cl,cAMP) leads to cell swelling. We first investigated the mechanism underlying the I_Cl,cAMP-independent recovery (shrinkage) of the swollen cells. They shrank when the membrane potential (V_m) was made negative to the equilibrium potential of Cl⁻ (E_Cl) by lowering [K⁺]_o or [Cl⁻]_o in the high-K⁺ solution. This shrinkage was attenuated by the inhibitors (DIDS, glibenclamide, furosemide) of swelling-activated Cl⁻ current (I_Cl,swell). These findings suggested an involvement of I_Cl,swell in the observed isosmotic cell shrinkage. On the other hand, an application of hyposmotic (70% of control) solution to the cells at normal [K⁺]_o (E_Cl>V_m) induced a cell swelling, and the swollen cells underwent a slight but definite spontaneous cell shrinkage during hyposmotic challenge, indicating the operation of the mechanism of regulatory volume decrease (RVD). This RVD was pronounced at low [Cl⁻]_o, at which E_Cl was much more positive than V_m. On the contrary, when the hyposmotic solution was applied to the cells at high [K⁺]_o, at which E_Cl was negative to V_m, the cells swelled vigorously and monotonically without showing RVD, the swelling being much greater than that seen at normal [K⁺]_o. Both the RVD at normal [K⁺]_o and the extra cell swelling at high [K⁺]_o were suppressed by DIDS. These results suggest that I_Cl,swell activated by cell swelling can shrink or inflate the cardiac cells under hyposmotic as well as isosmotic conditions, depending on V_m and E_Cl.