Programmed cell death includes apoptosis and programmed necrosis (AKA, necroptosis). A well-recognized feature of necrosis is the loss of membrane integrity of the dying cell. Cells undergoing apoptosis, however, maintain their membrane integrity. We previously reported massive cardiomyocyte (CM) necrosis, before increased apoptosis is discernible, in mouse hearts with cardiomyocyte-restricted knockout of the COP9 signalosome subunit 8 (CR-Csn8KO). This is associated with defective autophagosome maturation and ubiquitin-proteasome system (UPS) dysfunction. Consequently, mice with perinatal CR-Csn8KO develop rapidly dilated cardiomyopathy (DCM) and die prematurely. The present study sought to search for the mechanisms underlying the CM necrosis. Here our immunoprecipitation revealed significant increases of RIP1-interacted RIP3 in CR-Csn8KO myocardium, indicative of activation of the RIP1-RIP3 pathway. The RIP1-RIP3 pathway is known to mediate necroptosis. To further test whether RIP1 activation plays an essential role in CM necrosis in CR-Csn8KO mice, we treated the mice with a RIP1 kinase specific inhibitor necrostatin-1 (Nec-1) or vehicle control via osmotic mini-pump implanted in the peritoneal cavity at 2 weeks of age. Nec-1 significantly suppressed CM necrosis as measured by the positivity of Evan’s blue dye (EBD) uptake (p<0.00001), prevented left ventricle dilatation (p<0.05) at 3 weeks, and delayed premature death of the CR-Csn8KO mice (p=0.0072). These results demonstrate that CM necrosis in CR-Csn8KO mice is necroptosis in which the RIP1-RIP3 kinases-mediated pathway plays a major pathogenic role, and that CM necroptosis is the primary cause of DCM and mouse premature death. Increased mitochondrial membrane permeability, which can be suppressed by inhibition of cyclophilin D, has been shown to play a critical mediating role in myocytes necrosis. Hence, we cross-bred CR-Csn8KO mice with cyclophilin D knockout mice and obtained the Kaplan-Meier survival curve. Unexpectedly, cyclophilin D deficiency exacerbated the premature death of CR-Csn8KO mice (p=0.007), indicating that cyclophilin D regulated mitochondrial membrane permeability does not play an important role in CM necrosis in CR-Csn8KO mouse hearts.