This study examined the role of Ca2+ in regulatory volume decrease by Necturus erythrocytes. Hypotonic shock (50% tonicity)stimulated an increase in cytosolic free Ca2+, detected using epi-fluorescence microscopy and the fluorescent Ca2+ indicator fluo-4-AM (10 μM). A similar increase in fluorescence did not occur under isosmotic conditions, unless cells were exposed to the Ca2+ionophore A23187 (0.5 μM). In addition, a low Ca2+ medium(amphibian Ringer solution with 5 mM EGTA), hexokinase (2.5 U/ml, an ATP scavenger), suramin (100 μM, a P2 receptor antagonist) and gadolinium (10μM, a stretch-activated channel blocker) each inhibited the swelling-induced increase in Ca2+. Consistent with these studies, a low Ca2+ Ringer solution increased osmotic fragility, whereas volume recovery following hypotonic shock (measured with a Coulter counter)was potentiated with A23187 (0.5 μM). By contrast, a low Ca2+extracellular medium or buffering intracellular Ca2+ with BAPTA-AM(100 μM) reduced the rate of volume recovery following hypotonic challenge. Finally, a low Ca2+ extracellular Ringer solution inhibited whole-cell currents that are activated during cell swelling (measured with the whole-cell patch clamp technique). Our results are most consistent with hypotonic shock causing an increase in cytosolic free Ca2+, thereby stimulating subsequent volume decrease.