The rates of synthesis (in per cent per day) of the total protein (ks) and myosin heavy chains (ke HC) and actin (ke A), isolated by preparative electrophoresis from myofibrils prepared in a relaxing medium, have been measured in rat heart ventricles after a continuous infusion of 50 μCi of (¹⁴C) Tyrosine (0.5 ml/h during 6 h). Normal values, in the sham-operated group, were for ks 19%, ke HC 23% and ke A 11%. Two to 4 days after an abdominal aortic stenosis there was an increase in the specific radioactivity of free plasma and intra-cellular tyrosine and of protein-bound tyrosine. The rate of synthesis also increased up to 37% for ks, 41% for ke HC and 21% for ke A but the ratio ke HC/ke A remained unchanged. We conclude that the normal heterogeneity of the rate of synthesis of the two main contractile proteins, at least when measured on myofibrils free from easily releasable myofilaments, was unmodified by cardiac overload, although both of these rates of synthesis were stimulated.