Concentration jumps of intracellular ATP were produced by photolysis of P3-1-(2-nitrophenyl)ethyl (NPE)-caged ATP and were used to investigate the passive relengthening properties in unloaded cardiac myocytes. Patch-clamp pipettes in the whole-cell mode were used to voltage-clamp the myocytes and to load the cells with caged ATP while optical methods were applied to record sarcomere length or cell length simultaneously. Cell length was varied using energy deprivation contractures while intracellular Ca2+ was controlled with EGTA. At sarcomere lengths between 1.8 and 1.4 microns cellular relengthening after photolysis of caged ATP was rapid (t1/2 approximately 100 ms) and could be well described by a simple mechanical model. However, ATP jumps made at sarcomere lengths approximately 1.1 microns led to slow relengthening (t1/2 approximately seconds), comparable to the slow reextensions observed in skinned myocytes after bulk solution changes. We attribute the slow and incomplete relengthening of intact and skinned myocytes after severe rigor shortening to deformation and alteration of structural elements inside the cell. Relengthening from intermediate sarcomere lengths in intact cells is elastic and provides information about the underlying relengthening forces inside the cell. The data do not support the presence of a significant discontinuity in elastic modulus at a sarcomere length of approximately 1.6 microns expected from ultrastructural features of the sarcomeres and from observations in skinned myocytes. Our results suggest that the cell length measurements usually performed in this preparation provide an adequate description of the force produced by the unloaded cell in the steady state. The results also provide a way to estimate the error arising from viscous forces during rapid shortening.