Contractures can be evoked from frog atrial trabeculae by raising extracellular pH in Na-free fluid after the spontaneous relaxatin of the tension evoked by removing extracellular Na+. These alkalinity contractures are unaffected by local anaesthetics but are absent following a brief perfusion with Ca-free fluid but are partially inhibited by local anaesthetics. Perfusion by solution containing a high Pco2 fails to induce contractures. The relationship between [Na]0 and tension is altered by changing extracellular pH; acidic fluids below pH 6.0 reduce tension and alkaline fluids increase tension over the whole range. NH4Cl has the same effect as alkaline fluids while CO2-containing fluid has no effect. These results can be interpreted if the effects of variation of extracellular pH, on Na-withdrawal contractures, are mediated by changes in intracellular pH which in turn affect the Ca-sensitivity of the contractile proteins.