Adult female Sprague-Dawley rats, age 10 months, were used to estimate the minimum dietary lysine requirement for tissue maintenance. Ten animals were assigned to one of eight treatment groups by weight. The dietary lysine levels ranged from 0.06 to 0.36 % of diet and the feeding period lasted 56 days. Carcass and liver nitrogen and total serum proteins were determined, and a dietary lysine requirement was estimated from the data obtained. Carcass and liver analysis included weight, total nitrogen, percent protein, percent water and percent fat. Using a one-way analysis of variance, results showed no significant differences in carcass or liver composition between the treatment groups. The data indicated that the mature female rat has a dietary lysine requirement lower than 0.06 % of diet, or less than 20.1 mg/day/kg0.75. Since previous investigators used a protein based diet, a possible cause for the insignificant differences between values seen in this study may be a consequence of using a nonprotein, amino acid mix base supplemented with lysine. In future studies for determining the dietary lysine requirement in the adult female rat, dietary lysine levels below 0.06 % of diet must be included when using a nonprotein based diet. / M.S.