Rates of protein turnover were investigated in a working rat heart preparation. The glucose perfused working heart was characterised with respect to aortic and coronary flow, cardiac output, lactate release, glucose uptake and the effect of insulin on these processes at a number of different afterloads and preloads. Protein synthesis r u 1 was measured by incorporation of [U-14C] phenylalanine into atrial and ventricular protein. Protein degradation was measured by following the release of phenylalanine in the presence of cycloheximide. Rates of protein synthesis were linear and maximal with respect to perfusate amino acid concentration. Atrial protein synthesis rates and RNAccnotJoVwhcrfc were twice those in the ventricles. The efficiencies of atrial and ventricular protein synthesis (protein synthesis rate relative to RNA) were similar. Insulin stimulated protein synthesis in both atria and ventricles. Synthesis of an actomyosin fraction in atria was also about twice that in the ventricles. The effects of workload and insulin on protein turnover were investigated. Protein degradation rates were linear and equal at all workloads including increased afterload and preload. Insulin inhibited protein degradation at all workloads but the inhibition was greatest under conditions of increased afterload. Increased left atrial filling pressure did not change the rate or efficiency of ventricular and right atrial protein synthesis but the rate and efficiency of left atrial protein synthesis were elevated.