Lysosomes are presumed to be involved in protein degradation in heart, but their exact role is poorly understood. Several interventions that are known to alter cardiac proteolysis (e.g., insulin) also produce lysosomal changes that might account for the observed changes in protein degradation; but many other interventions appear not to do so. Agents that interfere with lysosomal function (e.g., sucrose, chloroquine, methyladenine, leupeptin) cause a 25% reduction in the rate of degradation of total protein in fetal mouse hearts in organ culture; however, in the same hearts the rate of degradation of myosin and other myofibrillar proteins remains unchanged. Thus, it appears that lysosomes are involved in cardiac proteolysis, but may not play a rate-limiting or regulatory role in many circumstances. The regulation of proteolysis by insulin appears to involve non-lysosomal pathways in addition to any lysosomal alterations it may cause. Furthermore, the initial cleavage of myofibrillar proteins appears no to be dependent on normal lysosomal function.