To investigate the contribution of hepatic and peripheral tissues to the enhanced glucose disposal rate (Kg) observed in magnesium (Mg)-deficient rats, euglycemic-hyperinsulinemic clamps were performed with continuous infusion of [3-3H]glucose and three insulin infusion rates, 1, 8, and 16 microU.kg-1.min-1. Moderately Mg-deficient (Mg-, 4.2 microM Mg/g diet) and Mg-adequate (Mg+, 16.7 microM Mg/g diet) Sprague-Dawley rats were studied after 3 wk of dietary treatment. Growth, fasting glucose, and insulin concentrations were not affected by dietary treatment. Basal hepatic glucose output (HGO) and glucose disposal (Rd) were increased by 24% in Mg- rats (P < 0.001). After 1 microU insulin.kg-1.min-1 infusion, Rd and the glucose infusion rate that maintained euglycemia were significantly increased in Mg- rats by 24 and 46%, respectively. However, when the increase in Rd above baseline was examined, no significant differences were observed. Therefore, the increased basal glucose disposal observed in Mg- rats may be mediated by noninsulin-dependent mechanisms. Insulin suppression of HGO during 1 microU insulin.kg-1.min-1 infusion was greater in Mg- rats (43%) compared with Mg+ rats (27%, P < 0.05). In conclusion, the increased Kg observed in Mg- rats is likely to be caused by an increase in noninsulin-mediated glucose uptake and an enhancement of hepatic insulin sensitivity.