This study was undertaken in patients with heart failure to investigate the relation between plasma norepinephrine (NE) concentration and Mg dynamics. The study subjects comprised 16 patients with chronic heart failure (mean age 64.9+/-10.0 years). Cardiopulmonary exercise testing was performed on all patients, and anaerobic threshold (AT), peak oxygen uptake (peak VO2) and peak exercise time were measured. Resting and peak values of plasma NE concentration and serum and erythrocyte magnesium concentration were also measured. The results were as follows: the serum Mg concentration was increased significantly immediately after exercise (p<0.01), and the erythrocyte Mg concentration showed a tendency to decrease (p<0.1). The resting plasma NE level was inversely correlated with AT (p<0.05, r=-0.57), peak VO2 (p<0.05, r=-0.55) and peak exercise time (p<0.01, r=-0.62). When the plasma NE concentration at rest was analyzed in 2 groups of patients, ie, those with higher than average and those with lower than average concentrations, the resting erythrocyte Mg concentration was significantly lower in the high-NE group (2.2+/-0.3 mg/dl) than in the low-NE group (2.7+/-0.5 mg/dl) (p<0.05). The data indicate that patients with chronic heart failure associated with high NE levels at rest who showed low exercise tolerance have intracellular hypomagnesemia, which may be caused by Mg migration from intracellular to extracellular spaces.