It has been proposed that P glycoprotein (Pgp) expression is associated with swelling-activated Cl- currents in multidrug-resistant cells. The Pgp substrate vinblastine and the modulator verapamil produced a reversible concentration-dependent block of swelling-activated Cl- currents in both a drug-sensitive cell line (MCF-7) and a Pgp-expressing derivative (BC19/3). The similarity of the results obtained in both cell lines suggests that the mechanism of block is not related to Pgp expression and supports the hypothesis that Pgp expression is not necessary for the swelling activation of Cl- currents. In contrast to the results obtained with vinblastine, two other cytoskeleton-disrupting agents, colchicine and cytochalasin D, were not able to affect the swelling-activated Cl- currents in either cell line. The data provided no evidence for the involvement of the cytoskeleton in the swelling activation of Cl- channels in these cell lines. The Cl- channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid and 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid, each produced a similar reversible concentration-dependent block in the swelling-activated currents in both the Pgp-expressing and nonexpressing cells. This strongly suggests that the Cl- channel(s) responsible for the swelling-dependent current in both cell lines are the same and, since MCF-7 cells do not express Pgp, that Pgp is not the channel responsible for the volume-activated Cl- currents in these cells.