Recent studies in our laboratories and in the laboratories of other investigators suggest the presence of a sodium-potassium pump inhibitor in the plasma of some patients with arterial hypertension. We here review these studies in an attempt to determine what characteristics increase the likelihood of detecting the inhibitor. The review suggests but does not prove that the inhibitor is most likely to be found in males with increased sodium intake, decreased renal function, and decreased plasma renin activity. In future studies of plasma sodium-potassium pump inhibitory activity in hypertensive humans, we should pay more attention to the characteristics of the patients. These characteristics include age, sex, race, therapy, stage of hypertension, sodium intake, renal function, and renin status. We should also make an attempt to match the patients properly with normotensive control subjects.