We report characterization of a transgenic mouse that overexpresses constitutively active protein kinase Cepsilon in the heart and slowly develops a dilated cardiomyopathy with failure. The hemodynamic, mechanical, and biochemical properties of these hearts demonstrate a series of temporal events that mark the progression of the disease. In the 3-month transgenic (TG) animals, contractile properties and gene expression measurements are normal, but an increase in myofibrillar Ca2+ sensitivity and thin filament protein phosphorylation is noted. At 6 months, there is a decrease in the myofibrillar Ca2+ sensitivity, a significant increase in beta-myosin heavy chain mRNA and protein, normal cardiac function, but a blunted response to an inotropic challenge. The transition at 9 months is especially interesting because age-related changes appear to contribute to the decline in function seen in the TG heart. At this point, there is a decline in baseline function and maximum tension produced by the myofibrils, which is coincident with the onset of atrial myosin light chain isoform re-expression in the ventricles. In the 12-month TG mice, there is clear hemodynamic and geometric evidence of failure. Alterations in the composition of the myofibrils persist but the phosphorylation of myosin light chain 2v is dramatically different at this age compared with all others. We interpret these data to implicate the disruption of the myofibrillar proteins and their interactions in the propagation of dilated cardiac disease.