In 13 essential hypertensives, 15 renal hypertensives, 16 patients with phaeochromocytoma, 6 patients with renal artery stenosis and in 21 normotensive controls the Ca2+ and K+ content in the asked material from red blood cells was determined by flame photometry. Furthermore, intracellular K+ concentration was measured in the haemolysate. The ratio between Ca2+ and K+ concentration in ashed red blood cells (Cai2+/Ki+) was 2.07 +/- 0.9110-3 in normotensives, 4.58 +/- 1.68 10(-3) in essential hypertensives (p less than 0.01), 3.13 +/- 1.90 10(-3) in renal hypertensives (p less than 0.05), 2.15 +/- 1.62 in patients with phaeochromocytoma and in patients with renal artery stenosis 2.25 +/- 1.25. Intracellular K+ concentration was 94.3 +/- 3.1 mumol/l in normotensives, 94.7 +/- 3.8 mumol/l in essential hypertensives and 93.8 +/- 3.9 mumol/l in renal hypertensives, 198.2 +/- 149.1 mumol/l in patients with phaeochromocytoma and 201.1 +/- 139.1 mumol/l in patients with renal artery stenosis. Therefore, intracellular total Ca2+ concentrations is concluded to be increased in red blood cells from essential hypertensives and, to a lesser extent, in those from renal hypertensives. The method to determined Cai2+/Ki+ in the red blood cells may thus be suitable to assess cellular Ca2+ content in hypertension.