DNA polymerase delta, a fourth cellular DNA polymerase, might play an important role in cellular events. The properties of the enzyme suggested possible roles in nuclear DNA replication and DNA repair. By using specific assays that allow the determination of DNA polymerases delta and alpha in crude enzyme fractions (1), we show here that in developing rat neurons, which stop dividing before birth, both DNA polymerases delta and alpha drop sharply in an identical pattern from a high level with the approach of term and disappear at approximately three weeks of postnatal age. The results suggest that DNA polymerases delta and alpha might have a coordinated function during DNA replication in eukaryotic cells.