Changes in intracellular pH (pHi) are thought to produce large changes in force production in the uterus. There have however, been no simultaneous measurements of pHi and force in the uterus and therefore no direct information is available about the relation between the two. We have used carboxy-SNARF (a pH-sensitive fluorophore) in small strips of longitudinal myometrium and obtained simultaneous measurements of pHi and force. SNARF did not alter contractile function, and continuous measurements of pHi could be made for 2 hours. The mean resting pHi (7.16) was similar to that reported previously. Application of weak bases rapidly raised pHi, in a concentration-dependent manner, followed by a gradual restoration of pHi to resting levels. Alkalinization greatly increased the frequency of contractions, often accompanied by a small increase in their amplitude. Removal of base produced a rebound acidification which transiently abolished contractions. Direct acidification of the cytoplasm, by application of weak acid, also abolished contractions. However the alkalinization which accompanied removal of acid, produced variable effects on force.