To better understand the mechanism(s) whereby chronic adrenaline treatment rendered rats less susceptible to the hyperglycemic effect of this catecholamine sharing both α- and β-adrenergic activities, a similar treatment was done with isoproterenol, a pure β-adrenergic agonist (300 μg/kg daily for 28 days). The dynamics of plasma glucose, insulin, and glucagon were studied in unanesthetized control and treated rats during an isoproterenol infusion (0.75 μg kg−1 min−1), an acute intravenous glucose load (0.5 g/kg), or the simultaneous administration of both agents. Chronic treatment with isoproterenol did not modify the basal glucose and glucagon levels but it greatly diminished the insulin values (40.1 ± 3 vs. 59.6 ± 3.6 μU/mL, p < 0.01). In both groups, the isoproterenol infusion produced an increase in glucose concentration which was associated with a prompt rise in insulin levels; however, the glucose and insulin elevations were significantly lower in the isoproterenol-treated rats than in the control animals (p < 0.01). Despite these glucose and insulin increases, plasma glucagon concentrations similarly rose during the first 15 min of infusion in both groups, followed by a return to their basal levels. During the intravenous glucose tolerance test, the plasma glucose, insulin, and glucagon responses showed similar patterns in both groups of rats; however, during the concomitant isoproterenol infusion, the treated rats showed a better glucose tolerance than their controls. Similarities with results obtained with adrenaline treatment suggest that adaptation to the hyperglycemic effect of both catecholamines may be mediated by desensitization at the β-receptor level; furthermore, such a modification may lead to an increase in the sensitivity to insulin.