The effect of calcium on protein synthesis was studied in myocardial ventricular slices.Calcium in concentrations ranging from 0.01 to 4.0 mM increased the incorporation of leucine into proteins of all the subcellular fractions. Maximal protein synthesis occurred at concentrations of 0.5–2.5 mM calcium, and [14C]-leucine incorporation into proteins was linear for the 4-h incubation period only in the presence of calcium.In all experiments, the specific activity of [14C]-leucine in the sarcoplasm was not different either in the absence or in the presence of calcium. Optimal protein synthesis in the myocardium depends on the presence of calcium.