To determine whether norepinephrine could subserve a hormonal as well as a neurotransmitter function, norepinephrine was infused for 60 min into each of five normal young men in doses of 0.1, 0.5, 1.0, 2.5, and 5.0 microgram/min. After infusion, the plasma norepinephrine concentration fell with a mean (+/-SD) half-time of 2.4 +/- 0.7 min. The mean (+/-SD) norepinephrine metabolic clearance rate was 3,070 +/- 200 ml/min. The calculated basal plasma norepinephrine production rate was 0.7 microgram/min. The blood pressure and circulating glycerol, acetoacetate, beta-hydroxybutyrate, and glucose (increased) and the heart rate and circulating insulin, lactate, pyruvate, and alanine (decreased) exhibited highly significant parabolic relationships with the steady-state plasma norepinephrine concentrations. However, norepinephrine levels in excess of 1,800 pg/ml were required to produce hemodynamic and/or metabolic effects. Thus, under usual conditions, the biologic actions of norepinephrine can be attributed only to its sympathetic neurotransmitter function. Plasma norepinephrine concentrations do at times exceed 1,800 pg/ml during exercise and during major acute illness. Thus, under conditions of stress, norepinephrine may subserve a hormonal, as well as a neurotransmitter, function.