Changes in the expression pattern of DNA polymerase beta gene during rat lung, brain, and testis development have been investigated. A decrease in the level of beta-pol mRNA was observed during postnatal development of lung and brain. By contrast, an almost 20-fold increase in the level of beta-pol mRNA was observed during spermatogenesis. For most adult rat tissues the abundance of beta-pol mRNA was low compared with that of beta-actin mRNA. Northern blot analysis revealed four distinct transcripts hybridizing to beta-pol probes. At least two of them, 1.4 kb and 4.0 kb, were products of a beta-polymerase gene. The changes in the expression pattern during lung and brain development, and during spermatogenesis, suggest involvement of DNA polymerase beta in gap-filling DNA synthesis during recombination.