To investigate the volume regulatory ability of frog erythrocytes, we studied water and ion contents in the cells incubated in isosmotic (210 mOsm) and anisosmotic media. The cell volume decreased to 63% of the control in a hypertonic medium (+200 mM sucrose) and then gradually recovered to approximately 81% within 2 h. The regulatory volume increase (RVI) was associated with the accumulation of intracellular Na+, whereas the intracellular K+ content remained the same. Na+ (22Na) influx into the red cells increased from 1.1 +/- 0.2 in an isotonic medium to 12.4 +/- 0.4 mmol/(1 cells.h) in a hypertonic medium. The changes in both intracellular water and Na+ contents and the Na+ influx were blocked by 1 mM amiloride. Thus, it is likely that RVI observed in frog erythrocytes under hyperosmolar conditions is due to the activation of Na+/H+ exchanger. The exposure of the red cells to hypotonic media (approximately 160 mOsm) resulted in an increase of cell volume to 138% of the control value. The swollen cells gradually recovered their volume (on average by 69% in 2 h) due to losses in intracellular K+ and water. K+ (86Rb) influx in erythrocytes in the hypotonic medium (3.4 +/- 0.3 mmol/l.h) was significantly greater than that in the isotonic medium (1.4 +/- 0.03 mmol/l.h). K+ loss from frog erythrocytes incubated in a K(+)-free medium was significantly stimulated by hypotonic swelling, despite a decrease in the intracellular in K+ concentration. The regulatory volume decrease (RVD) and the changes in K+ transport across the red cell membrane in a hypotonic medium were completely abolished after C1- replacement by NO3-. The Cl(-)-dependent K+ loss in a hypotonic medium was partially (approximately 50%) inhibited by 0.05 mM DIOA ([(dihydroindenyl)oxy]alkanoic acid) and 0.5 mM quinine, but it was unaffected by 0.05 mM bumetanide. These results indicate that the swelling-activated K+, Cl- cotransport is involved in the RVD in frog erythrocytes.