Recent studies have shown that the KATPase of porcine cardiac S-1 is severalfold stronger than Kbinding. As with skeletal S-1, the four-state model can only explain this observation with the assumption that the release of the products of hydrolysis is rapid and not rate limiting. However, if the release of products is fast, the four-state model predicts that the extent of oxygen exchange with porcine cardiac S-1 should fall toward zero at high actin concentrations, as previously observed with skeletal acto-S-1. In the current work, we show that, in fact, the extent of oxygen exchange for porcine cardiac S-1 remains significant even at infinite actin concentration (i.e., with cross-linked actin-S-1) and that, therefore, the four-state model cannot adequately account for the oxygen exchange data and the ratio of Kbinding to KATPase simultaneously. As in the skeletal case, in order for the six-state model to account for these data, it is necessary to assume that Pi rotation in the acto-S-1.ADP.Pi state is rate limiting for oxygen exchange.