Possible interactions between the effects of pH and phosphate (Pi) on the maximum force development of cardiac myofibrils were investigated in rat skinned trabeculae in solutions of different pH (7.4-6.2) and [Pi] (where  denote concentration). At pH 7.0 there was an inverse linear relationship between force and log [Pi] over the [Pi] range 0.2-20 mM; its slope (-0.46/decade) was twice that found previously for skeletal muscle . Acidosis depressed force substantially, but the relative change of force was unaffected by Pi addition (0, 5, 20 mM); there was no evidence for the synergism between acidosis and Pi that would be expected if some of the inhibition by acidosis was due to protonation of Pi to the putative inhibitory form, H2PO4-. It was taken into account that even without Pi addition, there was enough Pi inside the muscle from various sources to produce significant changes in [H2PO4-] as the pH was varied. The results suggest that H+ and Pi inhibit maximum force development of cardiac myofibrils independently, by different mechanisms. From this it is argued that H+ and Pi may be released at different steps in the crossbridges cycle. In the myocardium Pi and H+ probably exert tonic inhibitory influences on cardiac myofibrils under all conditions.