Inorganic phosphate, which increases in the hypoxic cardiac cell, depresses force development. The cardiac muscle of freshwater turtle maintains a remarkably high contractility during hypoxia; this may involve a low sensitivity to phosphate. Therefore, freshwater turtle and rainbow trout were compared with regard to Ca(2+)-activated force in skinned atrial trabeculae in a bath containing 3 mM ATP buffered by 15 mM creatine phosphate in the presence of creatine kinase. For turtle, an increase in phosphate from 0 mM to either 6 mM or 12 mM reduced maximal force by 50% and 80% respectively, whereas the Ca2+ activity eliciting half maximal force (Ca0.5) was increased by 70% in 6 mM and could not be reliably recorded in 12 mM. For trout, the effects of phosphate were less pronounced. An increase from 0 mM to 12 mM did not affect maximal force significantly, but elevated Ca0.5 by 70%. Hypoxia increases ADP as creatine phosphate is shifted to creatine, therefore, creatine phosphate was changed from 15 mM to 3 mM and creatine from 0 mM to 12 mM. After these changes, the elevation of phosphate from 0 mM to 12 mM had no significant effects for either turtle or trout. In conclusion, the high performance of turtle cardiac muscle during hypoxia does not involve a low sensitivity of the contractile system to phosphate. In addition, the effect of increased phosphate seems to be offset by a concomitant increase in ADP.