The effects of changing extracellular (pH_o) and intracellular pH (pH_i) on force and the mechanisms involved in the guinea pig portal vein were investigated to better understand the control of tone in this vessel. When pH_o was altered, the effects on force and calcium were the same irrespective of whether force had been produced spontaneously by high-K depolarization or by norepinephrine; alkalinization increased tone, and acidification reduced it. Because pH_o changes also lead to changes in pH_i, we determined whether the effects on force could be explained by these induced pH_i changes. It was found, however, that only with spontaneous activity did intracellular alkalinization increase force. In depolarized preparations, force was decreased, and, with norepinephrine, force was initially decreased and then increased. Thus the effects of pH_o cannot be explained solely by changes in pH_i. The role of the sarcoplasmic reticulum (SR) and surface membrane Ca²⁺-ATPase on the mechanism were investigated and shown not to be involved. Therefore, it is concluded that both pH_o and pH_i can have powerful modulatory effects on portal vein tone, that these effects are not identical, and that they are likely to be due to effects of pH on ion channels rather than the SR or plasma membrane Ca²⁺-ATPase.