A study was carried out in 15 male volunteers to evaluate qualitatively the secretion of growth factors following stimulation by oral amino acids. The results showed that oral administration of a combination of two amino acids (1200 mg 1-lysine plus 1200 mg 1-arginine) provoked a release of pituitary somatotropin and insulin. This phenomenon was reproducible and the growth hormone secreted in response to this stimulation had biological activity (as demonstrated by a radioreceptor assay and somatomedin induction). The effect appeared to be specific to the combination of the two amino acids; neither of the amino acids demonstrated appreciable stimulating activity when administered alone, even at the same doses.