The absorption of L-valine was studied in segments of the jejunum and ileum using a one pass in situ perfusion technique in 1-, 2-, 3-, and 4-wk-old healthy control and in growth-retarded rats (suckled with mothers fed a protein-deficient diet and fed the same diet after weaning). In the jejunum of control rats, rate of absorption of L-valine declined from about 270 to 80 mumol/h per g mucosal weight, between 2 and 4 wk of life. At each age period, in both segments, rates of absorption of L-valine based on weight of the segments were significantly greater in the growth-retarded than in the control rats. However, based on length of the intestinal segments rates of absorption were similar in corresponding segments of the control and growth-retarded rats. These findings indicated that although the intestine in the growth-retarded rats was atrophic compared to that in control rats, the capability to absorb L-valine had been preserved, by increasing the rate of absorption per unit weight of intestine.