Therapy with diuretics and/or digitalis, as well as the compromised cardiac function, contributes to electrolyte alterations (magnesium, potassium) in patients with congestive heart failure. We determined in 29 patients with heart failure (NYHA classes II-IV) the magnesium and potassium content in lymphocytes and in erythrocytes, as well as in 25 healthy subjects. In patients with heart failure, lymphocyte magnesium (4.4 +/- 0.6 fmol/cell) and lymphocyte potassium (50.4 +/- 6.0 fmol/cell) were only slightly (n.s.) lower compared to controls (Mg++: 4.7 +/- 0.6 fmol/cell; K+: 54.6 +/- 6.8 fmol/cell). Only in healthy subjects did we find a positive correlation between magnesium and potassium in lymphocytes (r = 0.83). The electrolyte content in erythrocytes was also not different in patients with heart failure (Mg++: 2.2 +/- 0.1 mmol/l; K+: 96.6 +/- 6.1 mmol/l) and controls (Mg++: 2.2 +/- 0.1 mmol/l; K+: 97.8 +/- 4.0 mmol/l). There was no correlation between the intracellular electrolyte content and the electrolyte levels in plasma, either for lymphocytes, or for erythrocytes in both groups. It is concluded that 1) determinations of magnesium and potassium in plasma do not reflect intracellular electrolyte content; 2) the magnesium and potassium content of lymphocytes and erythrocytes were not different in patients suffering from heart failure compared to controls; and 3) in heart failure, the relationship of magnesium and potassium in lymphocytes may be altered.