The circadian periodicity of plasma amino acids (AA) was explored using Cosinor analysis. Six healthy subjects eating a standard diet (30 Kcal and 1 g of protein/Kg of body weight) were studied. Plasma samples for AA determination were taken at 07:00, 11:00, 15:00, 17:00, 21:00 and 24:00. The majority of AA showed an increment in the evening samples. This 24-h variability led to a circadian rhythmicity for all AA, except for taurine, threonine, glutamate, alanine, cystine, and total tryptophan. On average, the fluctuation was approximately 15% of the mesor and the acrophase timing was between 17:00 and 22:00.