The sympathetic nervous system is one of the primary factors controlling vascular resistance. Vascular smooth muscle contraction by sympathetic stimulation is mediated primarily by alpha-adrenoceptors. In this study, we investigated the effects of different forms of adrenergic stimulation on simultaneously measured macro- and microcirculatory dynamics in the rat mesenteric vascular bed. Macrocirculatory effects were measured by means of registration of changes in mean arterial pressure (MAP) and blood flow feeding this tissue via a miniaturized Doppler flow probe around the superior mesenteric artery. Microvascular changes were measured at the level of A2 arterioles by means of intravital microscopic registration of arteriolar diameters and red blood cell velocity (RBC vel). Topical or intra-arterial application of noradrenaline caused a dose-dependent decrease in arteriolar diameter and RBC vel at doses that did not cause significant systemic effects. On the other hand, during intravenous application of noradrenaline or other vasopressor substances an increase in MAP and decrease in organ (Doppler) blood flow was not parallelled by a decrease in arteriolar diameter or RBC vel. In contrast, RBC vel increased in a dose-related manner, whereas arteriolar diameter only secondarily decreased to a small degree. Similar results were obtained when the sympathetic nervous system was activated by means of electrical stimulation of the posterior hypothalamus. These data indicate that adrenergic stimulation can elicit different vascular responses. The ultimate microvascular effect depends upon the way of stimulation or route of drug administration.