Background: Protein depletion is frequent in the elderly, but the underlying mechanisms are not yet fully understood. In particular, it is unknown whether there is a defect of adaptation to a restriction of food intake in the elderly. This study was performed to compare the effects of B-week dietary restriction (DR) on protein metabolism in both adult and aged rats, Methods: Adult (3-month-old) and aged (22-month-old) rats were acclimatized for 2 weeks and then fed a standard diet for 6 weeks, either ad libitum (control adult [C-Adult] and aged [C-Aged] rats) or with only 50% of the average intake of the second week of acclimatization (restricted adult [R-Adult] and aged [R-Aged] rats). Protein metabolism, in terms of tissue protein content, nitrogen balance, and 3-methylhistidine (3-MH) urinary excretion, was evaluated. Results: C-Adult rats gained 30.4% of initial weight, whereas the body weight (BW) of C-Aged rats was maintained. DR induced a rapid decrease in BW during the first 2 weeks in RAdult rats, but afterward BW remained stable. In R-Aged rats, BW loss was linear during the 6 weeks and significantly higher than for R-Adult rats (p < .01). In both restricted groups, muscle protein content was moderately affected by DR, whereas DR induced a marked decrease in visceral protein content. Nitrogen balance was decreased by DR but stayed positive in R-Adult rats, whereas it became null in R-Aged rats. Conclusions: In terms of protein metabolism, aged rats adapted less efficiently than adult rats to a long-term dietary restriction.