The difficulty of establishing a causal relationship between any measured phenomenon and hypertension is emphasized. The literature concerning ion movements in tissues from hypertensive patients and animals is briefly reviewed. It is pointed out that the evidence (i) is conflicting concerning changes of permeability of red blood cells in patients with hypertension; (ii) is more consistent in leukocytes; and (iii) is virtually nonexistent regarding ionic permeability of blood vessels from man. A few studies suggest that permeability to calcium is altered in aortae from hypertensive animals and that concentrations of calcium in platelets are correlated with blood pressure in man. Some of our previously published work on red blood cells is then summarized which suggests that altered passive permeability to calcium exists in red blood cells from spontaneously hypertensive rats. Experiments are described using the calcium antagonists D-600 or felodipine to further study previously reported differences in calcium handling in blood vessels from spontaneously hypertensive rats. We could detect no difference in the effects of these calcium antagonists on blood vessels from hypertensive animals compared with controls. It is suggested that the finding of increased passive permeability to calcium in tissues from hypertensive animals is a lead worth pursuing.