Exposure to hypotonic stress produces a transient increase in cell volume followed by a regulatory volume decrease (RVD) in both THP‐1 and HL‐60 cells. In contrast, cells exposed to hypotonic stress in a high K/low Na Hanks' solution not only failed to volume regulate, but displayed a secondary swelling. Thus, while an outward K gradient was required ful KVD, the secondary swelling indicated that hypotonic stress increased permeability in the absence of a negative membrane potential. The K channel blocker quinine (1–4 mM) blocked RVD in both cell types. Gramicidin's ability to overcome the quinine block of RVD indicated that RVD is mediated by a quinine‐sensitive cation transport mechanism that is independent of the swelling‐induced anion transport mechanism. Barium (1–4 mM), another K channel blocker, slowed the rate of RVD, while 4‐aminopyridine, charybdotoxin, tetraethylammonium chloride, tetrabutylammonium chloride, and gadolinium had no effect on RVD. Furthermore, RVD was not mediated by calcium‐activated conductances, since it occurred normally in Ca‐free medium, in medium containing cadmium, and in BAPTA‐loaded cells. Gramicidin produced little or no volume change in isotonic medium, suggesting that basal C1 permeability of both THP‐1 and HL‐60 cells is low. However, swelling induced an anion efflux pathway that is permeable to both chloride and bromide, but is impermeable to methanesulfonate and glutamate. The anion channel blocker 3,5‐diiodosalicylic acid (DISA) antagonized RVD in both cell types. In conclusion, RVD in THP‐1 and HL‐60 cells is mediated by independent anion and cation transport mechanisms that involve both a DISA‐sensitive anion pathway and a quinine‐inhibitable K efflux pathway, neither of which requires increases in intra‐cellular calcium to be activated. © 1994 wiley‐Liss, Inc.