There was a marked decrease in peptide synthesis using a renal cortical supernatant fraction obtained from hypophysectomized rats as a source of elongation factors, and this decrease in supernatant activity was reversed by the administration of growth hormone. The changes in kidney cortex differ from liver since the activity of the pH 5 supernatant, which provided elongation factors 1 and 2, was changed in kidney, whereas the activity of the ribosomes, rather than the supernatant, was changed in liver (Barden, N., and A. Korner, Biochem J127: 411, 1972). Changes in protein synthesis obtained with hypophysectomized rat kidney supernatant could not be attributed to changes in activity of ribonuclease, initiation factors, or mRNA, but rather to changes in the activity of aminoacyl-tRNA binding activity, since supplementing the kidney pH 5 supernatant of hypophysectomized rats with elongation factor 1, but not elongation factor 2, restored the incorporation of 14C-phenylalanyl-tRNA to the level of that observed using supernatant from control rats or growth hormone treated rats. Similarly, supplementing the pH 5 supernatant fraction of kidney obtained from hypophysectomized rats with elongation factor 1 restored the poly (U) directed binding of 14C-phenylalanyl-tRNA to the ribosomes to the level of that measured with kidney pH 5 supernatant obtained from control rats and growth hormone-treated rats. (Endocrinology93: 436, 1973)