The effect of digoxin (0.25 mg t.d.s.) given orally for 5 days, on sodium excretion, plasma renin activity, plasma aldosterone and blood pressure has been studied in 6 normotensive subjects, on a constant sodium intake. Average serum digoxin level measured in 5 subjects on the 5th day of treatment was in the upper range of the therapeutic level (1.92 +/- 0.3 nmol/l). Red cell sodium content increased from 9.75 +/- 1.2 mmol/l of cells in the control period to 15.3 +/- 1.7 mmol/l of cells on the 5th day of digoxin treatment when measured in 4 of the 6 subjects. In spite of this evidence of inhibition of the red cell sodium pump, there was no detectable change in sodium excretion, blood pressure, plasma renin activity and aldosterone.