Synthetic-adrenergic agonists and somatotropin are growth-modifying substances that speed up and occasionally improve the efficiency of protein deposition in lean tissues of cattle species. A comparatively little increase in the protein synthesis rate is what causes the ST-induced increase in muscle protein deposition. This may be mediated by the endocrine effects of substantial increases in blood levels of circulating IGF-I and other STdependent IGF system components; mediation by locally expressed IGF-I may also take place. In animals given -AA treatment, greater muscle protein accretion appears to be directly mediated by the synthetic agonist’s binding to muscle -1 or -2 receptors, which results in increased muscle protein synthesis and may be followed by decreased protein breakdown.