The influence of aging on the stability of mRNA has been investigated in Drosophila by measuring the incorporation of 3H-leucine into thorax and ovary proteins after injection of actinomycin-D. In the ovary, there is an identical decrease at any age in the rate of protein synthesis following injection of the drug. Thus, no age-related difference can be found in the stability of the corresponding mRNA templates, whose half-life is about 8–9 h. In the thorax, however, actinomycin-D significantly stimulates 3H-leucine incorporation in young flies, but slightly inhibits protein synthesis in old individuals. Thus, aging involves a progressive loss of stability of the mRNA templates responsible for the synthesis of thorax proteins. Flies of various ages have also been pulse-labeled with 3H-leucine in order to measure the rate of protein synthesis. Incorporation into ovary proteins exhibits, in function of age, a pattern of variation which follows the egg-laying curve, with a maximum early in adult life. Incorporation into thorax proteins, on the other hand, steadily increases with age. Similarly, the decrease of radioactivity in thorax proteins following a single injection of 3H-leucine is more rapid in old individual