The effect of acidosis and alkalosis on vascular smooth muscle contractions evoked by noradrenaline was studied. Helical strips of rabbit aorta were mounted for isometric tension recording. Acidosis (pH 7.24–6.51) was obtained by either increasing the Pco2(hypercapnic) and/or lowering the HCO3-concentration (hypobicarbonatic). Acidosis shifted the noradrenaline concentration-response curve to the right in a competitive manner. The maximal developed tension was unchanged atpH 7.24–6.90 and decreased by 30% atpH 6.51. Alkalosis (pH 7.61–8.04) did not alter nor-adrenaline-evoked contractions. The results suggest that hydrogen ions during acidosis (pH < 7.40) but not during alkalosis (pH > 7.40) exert α-adrenoceptor blocking properties.