The relationship between magnesium levels and oxygen uptake in patients with non-insulin dependent diabetes mellitus (NIDDM) without apparent visceral dysfunction was studied. Magnesium levels in plasma, erythrocytes and urine as well as oxygen uptake parameters were determined in NIDDM patients and controls. Oxygen uptake parameters were measured with an exercise ergometer and expired gas analysis according to Wasserman et al. Low oxygen uptake in NIDDM patients was correlated significantly with plasma and erythrocyte magnesium levels, but not with urinary magnesium excretion. In NIDDM patients, higher correlation coefficients were seen between oxygen uptake parameters at peak in men and at the anaerobic threshold in women and plasma or erythrocyte magnesium levels. These results confirm previous results indicating that plasma and erythrocyte magnesium levels in NIDDM patients are decreased, and further demonstrate that the decreased magnesium levels are positively correlated with oxygen uptake. Although the mechanism remains to be established, it is possible that the magnesium deficiency in NIDDM patients due to environmental or genetic factors may result in low oxygen uptake and decreased work capacity.