By using amino acid mixtures, a comparative study of nutritional effects of dietary essential amino acid proportions (EA %) has been made between growing and adult rats. Included were adult rats which were repletiog from an 8-day protein depletion. It was shown that for growing rats at least 55 EA % was required to attain the maxi mum values of growth, nitrogen balance, nitrogen balance efficiency (nitrogen balance/nitrogen intake) and protein efficiency ratio (PER). The maximum biological value was found to be 94 at the 50 EA % level. For adult rats, at least 40 EA % was required to gain the maximum values of nitrogen balance and nitrogen balance efficiency under both main tenance and repletion. The biological values were found to be nearly 60 and 80 for maintenance and repletion, respectively.