Effects of pH, PCO₂, and adenosine on the vascular contractility were investigated in the pig coronary arteries. The helical strips of isolated coronary arteries were immersed in the HEPES or HCO₃^-/CO₂-buffered Tyrode s solution equilibrated with 100% O₂ or 95% O₂-5% CO₂ at 35℃. The contraction was recorded isometrically using a force transducer. The amplitudes of contraction induced by ACh, high K⁺, and electrical Held stimulation (EFS) were decreased by elevating extracellular pH (pHo) and were increased by lowering pHo. A shift from 0% CO₂ to 5% CO₂ at constant pHo (pH 7.4) reduced the contractions induced by ACh, high K⁺, EFS. However the contraction induced by 100mM K⁺ was less influenced by the change of pHo or CO₂. The contraction induced by ACh in Ca²⁺free Tyrode s solution as well as the contraction developed by the addition of extracellular of Ca²⁺ were decreased by lowering pHo and were increased by elevating pHo. High K⁺ (25mM) induced contraction at pH 6.8 was not returned to the level of the contraction at pH 7.4 by the elevation of extracellular. calcium [Ca²⁺]_o. Adenosine-induced relaxation was more significant with 5% CO₂ than 0% CO₂ in the high K⁺-induced contraction and was more significant with low pHo than high pHo in the contraction induced by EFS. From the above results, it is suggested that H⁺ and CO₂ inhibit Ca²⁺ influx as well as Ca²⁺ release from intracellular Ca²⁺ storage sites and enhance the relaxing effect of adenosine in the pig coronary artery.