Objective: Acidosis is a very common pathologic process in perioperative management. However, how to correct severe acidosis to improve the efficacy of vasoconstrictors in hemodynamically unstable patients is still debated. The present study investigated whether severe extracellular acidosis influences the vasoactive properties of vasoconstrictors on human isolated arteries. Methods: Segments of intact distal internal mammary arteries were removed from 41 patients undergoing artery bypass grafting. The arterial rings were washed in Krebs-Henseleit solution and suspended in an organ bath. The rings were set at a pretension equivalent of 100 mm Hg, and the relaxation response to 10 μM acetylcholine was verified. Concentration-response curves for epinephrine, norepinephrine, methoxamine (α1A/D-Adrenoceptor agonist), phenylephrine (equipotent agonist of α1A/B-Adrenoceptors), and clonidine (α2-Adrenoceptor agonist) were achieved under control conditions (pH 7.40) and under acidic conditions by substitution of the Krebs-Henseleit solution with a modified solution. Results: Decreasing the pH from 7.40 to 7.20, 7.0, or 6.80 did not significantly alter the potency and efficacy of epinephrine and norepinephrine, although the standardized effect size was sometimes large. Severe acidosis (pH 6.80) did not significantly change the potency and efficacy of phenylephrine and clonidine, although it increased the efficacy and potency of methoxamine (P <.001 and P =.04 vs paired control conditions, respectively). Conclusions: Extracellular acidosis did not impair the vasoactive properties of epinephrine and norepinephrine in human medium-size arteries until pH 6.80. The results of the present study also suggest that acidosis might potentiate arterial responsiveness to vasoconstrictors, mostly by way of the α1D-Adrenoceptor. Copyright © 2014 by The American Association for Thoracic Surgery.