Previous results from this laboratory [Kao et al, (1976) Biochem. J. 154, 471-480] demonstrated that a low-molecular-weight DNA species, "sDNA", present in high concentrations during rapid larval growth of the yellow-fever mosquito, Aedes aegypti, may be an intermediate in DNA replication. We have now determined the life-span profiles of synthetic rates and the steady-state contents of sDNA and total DNA by radiometric and biochemical procedures and by using mosquitoes grown and maintained under defined and controlled conditions, including axenic culture and synchronization of stages. 2. Our results showed exponential increases in DNA contents and synthetic rates which paralleled the changes in weight during larval growth. Of special importance was the finding that in rapidly-growing larvae and newly-emerged adult mosquitoes high contents and rates of synthesis of sDNA were correlated with increases in total DNA/organism, which suggests that sDNA content may be an index of DNA replication. In senescent mosquitos a twofold increase in the synthetic rate of bulk DNA ("pDNA") without an increase in sDNA may reflect repair synthesis. 3. The significance of this investigation is that it may be the first quantitative and systematic description of the synthetic rates and contents of both DNA-replication intermediates and total DNA in an axenic, eukaryotic organism rather than a specific cell type. Equally important is that these findings describe DNA changes during growth and also during aging through senescence in an age-characterized organism.