The DNA polymerase activity in white matter, grey matter, and cerebellar regions of developing and aging rat brains was studied. The enzyme exhibited its highest activity during the early developmental stages with a decline to a low adult value by 225 days of age. However, the activity once again increased between 225 and 540 days, thus showing a second peak in the latter part of the life span. Studies with specific inhibitors like Aphidicolin and 2',3' dideoxy thymidine-5' triphosphate on the enzyme activity have revealed that this rise in the DNA polymerase activity in various regions studied was mainly due to an increase in the polymerase beta type. These results may explain the enhanced DNA content in aging rat brain observed earlier in this laboratory.