The in vitro effect of aldosterone on intracellular sodium and potassium concentration was investigated in human mononuclear leukocytes (HML). HML were separated from blood by a Percoll gradient and intracellular Na+ and K+ were determined before and after incubation for 1 h at 37 degrees C. The intracellular (ic) Na+ and K+ concentrations after separation of HML were 17 +/- 5 and 59 +/- 18 mmol/kg wet cells (mean +/- SD, n = 6), respectively. In the absence of aldosterone the ic Na+ concentration decreased to 12 +/- 4, whereas the ic Na+ concentration remained constant at 18 +/- 8 mmol/kg wet cells when aldosterone (1.4 nM) was added to the incubation medium. In parallel, the ic K+ concentration decreased without added aldosterone but remained constant with aldosterone. The effect of aldosterone on the ic Na+ and K+ concentrations of HML was blocked by the aldosterone antagonist canrenone (140 nM). Cortisol at a physiological concentration (40 nM) did not alter ic Na and K concentrations in these cells. The results suggest that aldosterone binding to specific receptors in human mononuclear leukocytes significantly contributes to the regulation of monovalent cation levels in these cells. Thus, this easily accessible model of HML allows study of the effects of mineralocorticoids in relation to their receptors in intact cells in normal and pathological states.