In young men ingesting protein meals, slowly digested proteins (caseins: CAS) induce a higher protein gain than those that are rapidly digested (whey proteins: WP). Our aim was to assess whether or not this is true in elderly men receiving mixed meals. The effects of meals containing either CAS or two different amounts of WP (WP-iN: isonitrogenous with CAS, or WP-iL: providing the same amount of leucine as CAS) on protein metabolism (assessed by combining oral and intravenous leucine tracers) were compared in nine healthy, elderly (mean +/- S.E.M. age 72 +/- 1 years) and six young men (24 +/- 1 years). In both age groups, WP-iL and WP-iN were digested faster than CAS (P < 0.001, ANOVA). Proteolysis was inhibited similarly whatever the meal and age groups (P = NS). Protein synthesis was higher with WP-iN than with CAS or WP-iL (P < 0.01), irrespective of age (P = NS). An age-related effect (P < 0.05) was found with postprandial leucine balance. Leucine balance was higher with CAS than with WP-iL (P < 0.01) in young men, but not in elderly subjects (P = NS). In isonitrogenous conditions, leucine balance was higher with WP-iN than with CAS (P < 0.001) in both age groups, but the magnitude of the differences was higher in the elderly men (P = 0.05). In conclusion, during aging, protein gain was greater with WP (rapidly digested protein), and lower with CAS (slowly digested protein). This suggests that a 'fast' protein might be more beneficial than a 'slow' one to limit protein losses during aging.