A cDNA probe encoding DNA polymerase beta (beta-pol) was used to study the level and size complexity of beta-pol mRNA in regenerating rat liver and other rat tissues. An almost 2-fold increase in beta-pol mRNA was observed 18-24 h after partial hepatectomy. In most adult rat tissues (liver, heart, kidney, stomach, spleen, thymus, lung and brain) the abundance of beta-pol mRNA was low. In contrast, young brain and testes exhibited beta-pol mRNA levels 5- and 15-times higher, respectively. The observed changes in the level of beta-pol mRNA in regenerating rat liver and in developing brain are correlated with reported changes in DNA polymerase beta activity. Four different (4.0, 2.5, 2.2, 1.4 kb) transcripts hybridizing to beta-pol probe were found in all tissues examined. The 4.0 kb transcript was dominant for young and adult brain, whereas the 1.4 kb transcript was dominant for testes. The significance of these transcripts is discussed.