Chromatin is organized into a repeating structure (nucleosome) made up of proteins and DNA. Micrococcal nuclease and DNAase I have been used to probe this structure in nuclear populations from three tissues (liver, brain, and heart) of the inbred mouse strain C57BL at different ages. For those parameters examined, for each tissue, chromatin contained essentially the same features of nucleosomal organization, regardless of the age of the mouse. Thus, the rate and extent of nuclease digestion and the size of the DNA repeat unit and nucleosome core are not significantly different as a function of age. However, the accessibility of internucleosomal DNA to micrococcal nuclease, as determined by measuring the DNA size distribution after nuclease cutting, may be partially limited in chromatin of brain (but not liver or heart) of older animals. These results indicate that there are no gross, age-related changes in the conformational state or organization of chromatin in these tissues. The results do not exclude smaller alterations in chromatin that might occur with age, which the current methodology might not be sensitive enough to detect.