1. The load bearing capacity during relaxation of ventricular cardiac muscle from various animal species was investigated. 2. The effect of load on the time course of relaxation was analysed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load (load clamps). 3. In heart muscle from the mammalian species studied relaxation was sensitive to loading conditions, whereas in frog heart muscle relaxation was largely independent of the loading conditions. The mechanical properties of relaxation of cardiac muscle appear, therefore, governed by the interplay of a load-controlled and an activation-controlled decay mechanism, the relative importance of which differs with species. 4. Load-dependence may be the mechanical expression of the ratio of the number of force generating sites at any time during contraction and relaxation to the load to be carried; this mechanism would predominate in mammalian animal species with a well developed calcium sequestering sarcoplasmic reticulum. Activation-dependence would seem to predominate in animal species, such as frog, in which calcium sequestration appears to be the rate limiting step during relaxation.