The effect of emetine hydrochloride on protein synthesis in rat myocardium was studied. Emetine inhibited incorporation of tritiated leucine into soluble proteins and actomyosin in an in vitro system using minced rat myocardium in a supporting medium. Fifty percent inhibition of incorporation of the isotope into soluble proteins and actomyosin occurred at an emetine concentration of 5 × 10-7M. Incorporation was also significantly inhibited when animals were treated with emetine for three days. The data suggest that emetine toxicity in the myocardium may be mediated through its effects on protein biosynthesis.