The effect of prolonged treatment with calcium channel blockers on potassium and magnesium stores is uncertain. We measured lymphocyte and serum magnesium and potassium in 28 patients treated for hypertension for 6 months with nifedipine or captopril. There was no difference in serum or lymphocyte concentrations in the two groups compared to 45 healthy, normotensive controls. These results suggest that intracellular cation levels are maintained with prolonged therapy with calcium channel blockers.