The role of contracture in the manifestation of calcium paradox-induced damage was examined using 2,3-butanedione monoxime (BDM) to inhibit myofibrillar activity. Calcium and sodium gain, loss of intracellular components, and changes in structure were monitored. If 30 mM BDM was added at the time of calcium repletion after 10 minutes of calcium-free perfusion, some protection was afforded, particularly at the early stages of calcium repletion. However, much greater protection was obtained if BDM was present during the final 2 minutes of calcium-free perfusion and throughout repletion. Sodium gain and loss of intracellular components were markedly attenuated, as was the incidence of severely contracted cells. Calcium gain, although significantly reduced during the period of repletion, was not abolished. After 10 minutes of repletion, a calcium content of 11.44 +/- 1.57 mumol/g dry wt was observed. This suggests that other noncontracture related routes of calcium entry are involved. If BDM is removed after 5 minutes of calcium repletion and perfusion is continued with BDM-free perfusate, there is a rapid gain of sodium, further gain of calcium, loss of intracellular components and the cells contract severely, tearing away from neighboring cells. It is evident, therefore, that returning calcium to hearts after a period of calcium-free perfusion under conditions that significantly reduce the typical calcium paradox-associated damage does not necessarily repair the underlying defect. These results support the hypothesis that contracture-induced sarcolemmal disruption may be responsible for the terminal manifestation of the calcium paradox.