In vitro binding of aldosterone to mineralocorticoid receptors on human mononuclear leukocytes (HML) and its effects on the intracellular sodium and potassium concentrations of HML have already been described. In the present paper this easily accessible human cell model was investigated with regard to the regulation of the cell volume by aldosterone, since the concordant changes of sodium and potassium were expected to be accompanied by water and volume shifts. As determined by the measurement of cell diameter and the planimetric estimation of cell area in photographs, cell volume decreased by approximately 16% when cells were incubated in RPMI-1640 medium without aldosterone added for 1 h at 37 degrees C, a decrease not seen when 1.4 nM aldosterone was added to the incubation medium; the effect was half maximal at a concentration between 0.07 and 0.14 nM. One hundred forty nanomoles canrenone antagonized the action of aldosterone, but cortisol was ineffective. The results indicate concordant changes of intracellular sodium and potassium and cell volume, if studied under the same conditions. These data are the first to demonstrate that aldosterone is a major physiological determinant of lymphocyte volume in isotonic media.