The effect of ouabain on the histamine secretion induced by compound 48/80 has been studied using rat peritoneal mast cells. Ouabain did not modify histamine release in the presence of millimolar concentrations of extracellular calcium. However, when mast cells were previously washed with a calcium-free buffer, ouabain strongly potentiated histamine release elicited by compound 48/80. The full potentiation of mast cell secretion by ouabain required 30 min preincubation before adding compound 48/80. It was inhibited by lanthanum and EGTA. Potassium deprivation mimicked the effect of ouabain. A 30 min preincubation time without potassium was also required. Potassium concentrations below 2.7 mM increased the effect of ouabain whereas higher potassium concentrations reversed this effect. The potentiation of compound 48/80-induced histamine release by ouabain or potassium deprivation was not immediately reversed by washing away ouabain or by adding potassium, respectively. The data confirm that sodium-potassium ATPase is involved, through a calcium-dependent process, in the regulation of histamine release from mast cells.