We evaluated the 75-g oral glucose tolerance test (OGTT)-induced modifications in glucose, insulin, and norepinephrine plasma concentrations, and in plasma, erythrocyte, and platelet magnesium levels in two groups of obese subjects (normotensive obese, NT-Ob, N = 19; hypertensive obese, HT-Ob, N = 15), and in a group of healthy control subjects (N = 12). During OGTT we detected a reduction in plasma magnesium concentrations and an increase in erythrocyte and platelet magnesium levels in the controls, whereas in both normotensive and hypertensive obese subjects, there was a reduction in plasma, erythrocyte, and platelet magnesium levels. Furthermore, no statistically significant difference was detected among the groups studied as regards delta-plasma magnesium. On the other hand, delta-erythrocyte magnesium and delta-platelet magnesium were negative in the NT-Ob (delta-erythrocyte magnesium: -0.24+/-0.08 mmol/L; delta-platelet magnesium: -0.49+/-0.09 micromol/10(8) cells) and HT-Ob (delta-erythrocyte magnesium: -0.20+/-0.10 mmol/L; delta-platelet magnesium: -0.50+/-0.11 micromol/10(8) cells) groups, and positive in control subjects (delta-erythrocyte magnesium: 0.40+/-0.08 micromol/L; delta-platelet magnesium: 0.47+/-0.09 mmol/ 10(8) cells). Finally, a direct correlation was found between delta-norepinephrine and delta-erythrocyte magnesium (r = 0.80, P < .01) in the control group, and a negative correlation was detected between delta-norepinephrine and delta-platelet magnesium (r = -0.58, P < .05) in the HT-Ob group. Our results seem to indicate that the insulin resistance status, the hyperglycemia, and the disregulation of the adrenergic system in obese subjects could be involved in the pathogenesis of the magnesium homeostasis impairment observed in the obese subjects.