Inbred strains of mice can be categorized into two groups based on the absence or presence of a duplicated copy of the renin structural gene; one-gene strains carry a single renin gene (Ren-1), whereas two-gene strains carry two renin genes (Ren-1 and Ren-2). To investigate the contribution that each locus makes to the composite levels of renin mRNA observed to accumulate in different tissues of two-gene strains, we have developed two assays capable of distinguishing the highly homologous Ren-1 and Ren-2 transcripts. Both methods take advantage of established base sequence differences between Ren-1 and Ren-2 coding regions by using reverse transcriptase-mediated primer extension of oligonucleotide primer/mRNA hybrids in the presence of appropriate deoxy-and dideoxynucleotide phosphates. Using these techniques we found that Ren-1 and Ren-2 mRNAs accumulate in the kidney of two-gene strains to approximately equal levels. These observations are discussed in light of potential mechanisms regulating the tissue-specific expression of the Ren-1 and Ren-2 loci.