In 16 normal subjects the circadian rhythm of ACTH has been studied during normal calories diet and after a 15 days period of high protein content diet (2 g/Kg body weight). The statistical study, according Cosinor method, has shown a significant increase of the mesor and of the amplitude, but has not shown any change of the ACTH and Cortisol rhythm, after hyperproteic diet. Data advise the increase of the tonic and fasic secretion of both hormones and shown the mantained acrophase. The action of the protein on the ACTH and Cortisol secretion does not seem related to mechanism like stress, neither to the probable mediation of intestinal like-ACTH messengers. On the contrary it seems related to a direct stimulus on the diencephalo-pituitary axis; it is possible that some amino-acids (tryptophan, arginine) act as a mediator, even if data concern just the effect of the over mentioned amino-acid in large doses.