Nine malnourished and nine children who had recovered from malnutrition were given a single injection of albumin-(131)I and were studied during consecutive periods in which the dietary protein was changed.Malnourished children had significantly lower catabolic rates of albumin than had recovered children on the same protein intake. Both nutritional groups, however, showed a progressive fall in catabolic rate after 3-5 days on a low protein diet (0.7-1.0 g/kg per day), and the maximum effect was seen in the 2nd wk of low protein feeding. The catabolic rate could return to normal within 3 wk in a malnourished child fed 4 g of protein/kg per day. The albumin synthetic rate was measured by a computer technique suitable for nonsteady-state conditions. The synthetic rate in the malnourished groups (101 mg/kg per day) fed on a low protein diet was significantly lower than the rate in the recovered groups (148 mg/kg per day). The synthetic rate responded rapidly to a change in diet; when the rate fell, the intravascular albumin mass was maintained by two compensating mechanisms: (1) a net transfer of extravascular albumin into the intravascular pool; and (2) by a delayed fall in the catabolic rate. The net transfer of albumin into the intravascular compartment diminished as the catabolic rate fell. ADAPTATION TO A LOW PROTEIN DIET WAS ASSOCIATED WITH: (a) low synthetic and catabolic rates of albumin; (b) a reduced extravascular albumin mass; and (c) a capacity for a rapid return to normal in the synthetic rate when the dietary protein was increased.