We investigated the effect of pH change on octapeptide cholecystokinin's ability to contract guinea pig gallbladder strips. In the absence of exogenous drugs, pH changes had no demonstrable effect on gallbladder tension. However, when gallbladder strips were contracted with CCK-OP at pH 7.3, increasing the pH in the muscle bath to 7.8 produced further increases in tension at each dose studied (P less than 0.025). Subsequently, decreasing the bath pH to 6.9 decreased the strip tension to less than that observed at pH 7.3 (P less than 0.025). Reversing the order in which these pH alterations were made produced similar results; a pH decrease from 7.3 to 6.9 decreased the response to CCK-OP at the two highest doses (P less than 0.025), while subsequent elevation of pH raised the tension to levels greater than those observed at pH 7.3 (P less than 0.01). Furthermore, contracting the strips at pH 8.0 and slowly decreasing pH at a rate of 0.1 pH units per minute consistently produced an acceleration of tension decay as pH fell. After contracting the strip at pH 6.7, slow increases in pH reversed the tension decay and enhanced the contractile response as pH was increased from 6.7 to 8.0. These results demonstrate that pH changes alter the gallbladder contractile response to cholecystokinin. Although this phenomenon has been reported for a number of other physiologic agents, we believe this is the first such demonstration for a peptide hormone.