The nature of the pathway for K⁺ release activated during regulatory volume decrease (RVD) in A6 epithelia was investigated by measuring cell thickness (T_c) as an index of cell volume and by probing K⁺ efflux with ⁸⁶Rb as tracer for K⁺ (R_Rb). Cell swelling was induced by sudden reduction of basolateral osmolality (from 260 to 140 mosmol/kgH₂O). Experiments were performed in the absence of Na⁺ transport. Apical R_Rb was negligible in iso- and hyposmotic conditions. On the other hand, osmotic shock increased basolateral R_Rb (R^bl_Rb) rapidly, reaching a maximum 7 min after the peak in T_c. Quinine (0.5 mM) completely inhibited RVD and R^bl_Rb. Also verapamil (0.2 mM) impeded volume recovery considerably; lidocaine (0.2 mM) did not exert a noticeable effect. The K⁺ channel blocker Ba²⁺ (30 mM) delayed RVD but could not prevent complete volume recovery. Cs⁺ inhibited RVD noticeably at concentrations <40 mM. With large Cs⁺ concentrations (>40 mM), the initial osmometric swelling was followed by a gradual increase of T_c, suggesting activation of Cs⁺ influx. Chronic exposure of the basolateral surface to 0.5 mM La³⁺ or Gd³⁺ completely abolished RVD and R^bl_Rb. Acute administration of lanthanides at the time of osmolality decrease did not affect the initial phase of RVD and reduced R^bl_Rb only slightly. Apical Gd³⁺ exerted an inhibitory effect on RVD and R^bl_Rb. The effect of Gd³⁺ should therefore be localized at an intracellular site. The role of Ca²⁺ entry could be excluded by failure of extracellular Ca²⁺ removal to inhibit volume recovery. In contrast to lanthanides, chronically and acutely administered Mg²⁺ (0.5 mM) inhibited RVD and R^bl_Rb by ~50%. These data suggest that K⁺ excretion during RVD occurs through a rather poorly selective pathway that does not seem to be directly activated by membrane stretch.