In isolated rhesus eccrine clear cells, regulatory volume decrease (RVD) occurs after osmotic swelling. RVD was completely inhibited by 1 mM quinidine, 200 nM charybdotoxin, 1 mM diphenylamine-2-carboxylic acid (DPC), or 0.1 mM 4-nitro-2(3-phenylpropyl-amino)benzoate. RVD was also inhibited in Ca(2+)-free medium by vinblastine (antimicrotubular agent), N-(6-aminohexyl)-5-chloro-1- naphthalenesulfonamide (W-7), or 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Valinomycin reversed quinidine- and DIDS-induced inhibition of RVD but not the inhibition caused by Ca(2+)-free medium, DPC, vinblastine, or W-7. The cytosolic free Ca2+ concentration, as determined by the fura 2 method, increased from 220 nM in the control to 435 nM during RVD. Activation of both K+ and Cl-currents was also directly demonstrated with the whole cell current-voltage clamp method. DIDS inhibited swelling-induced K+, but not Cl-, currents and depolarized the membrane potential during RVD, further supporting the notion that DIDS inhibited swelling-activated K+, but not Cl-, pathways. We conclude that the observed RVD is mediated by the activation of conductive Ca(2+)-dependent K+ and Cl- pathways.