Methylation of cytidine residues of DNA appears to be involved in the control of gene expression; therefore, hypomethylation of the DNA can be considered to be an active rather than a passive process. Previous studies of mammalian DNA methylation during aging have produced an assortment of results. In this study, we have examined the change in DNA base composition, including the change in 5-methyldeoxycytidine (m5dC) contents with age of mice. Livers pooled from 6 mice from each of six age groups between 6 and 31 months have been subjected to a sensitive analytical technique (HPLC). The DNA composition of different age groups is very consistent in most aspects. The ratio of (dA + dT)/(dG + dC + m5dC) as well as the sum of dC and m5dC remain constant throughout the animal's lifespan. However, a consistent gradual decline in m5dC content is noted as the age increases to 24 months. Thus, the 6-month-old animal pool exhibits the largest amount of m5dC (1.67 +/- 0.2%), which is reduced consistently as the animal's age reaches 24 months. This decrease in m5dC is accompanied by an increase in dC. No further decrease in m5dC occurs after 24 months; in fact, the data could indicate an increase after that age. No dTs are apparently produced by deamination of m5dC.