The demonstration of a species specificity of growth hormone (GH) made it possible to examine the metabolic action of this hormone in man by the use of human growth hormone preparations. The results obtained in our trials will be presented under the subheadings: protein metabolism, calcium metabolism, sodium and chloride metabolism, fat metabolism, ketogenesis, and finally carbohydrate metabolism. Protein metabolism. Human GH caused protein anabolism as shown by retention of nitrogen, phosphorus and potassium in proportions similar to those found in protoplasm. Retention of these elements appeared promptly upon GH administration and persisted during the whole treatment period. Upon withdrawal of GH the balances of nitrogen, phosphorus and potassium became negative, but the losses during the post-treatment period were smaller than the gains under GH. Previous pathological creatinuria disappeared, and NPN concentration in blood decreased on GH administration. Calcium metabolism. Increased calciuria appeared in all instances and the calcium balance