This study examined the age-dependent alterations in the in vivo incorporation of labeled amino acids into protein during the adult life spans of males and females obtained from genetically based long-lived and control strains of Drosophila melanogaster. All four groups tested showed significant decreases (ca. 50%) in the uptake of labeled amino acids as a function of age. Each of the four groups had their own characteristic temporal pattern of functional decrement. Both the long-lived and the control females have similar patterns of amino acid incorporation, but the onset of these changes is delayed by 10 to 20 days in long-lived animals. These alterations in protein synthesis appear to be related to corresponding changes in the female fecundity patterns of each strain. The male patterns differ from one another and from the female patterns, but they both show periods of high fluctuation early in life followed by a terminal period of relatively low and constant synthesis. These data are consistent with the view that the overall alterations in the longevity of these two strains are likely due to changes in the timing of particular events in the adult life cycle.