The quantitative measurements of aldose reductase (AR) and sorbitol dehydrogenase (SDH) gene expression in various rat tissues were performed by the competitive reverse transcription-polymerase chain reaction (RT-PCR). AR mRNA was detectable in all tissues analyzed with pronounced differences in the amounts. SDH mRNA was most abundant in testes and liver, but was absent from lens. The estimation of the AR cDNA/SDH cDNA ratio showed that the relative abundance of SDH to AR differs among tissues. These results indicate that different tissues contain varying amounts of AR mRNA and SDH mRNA; that is, each tissue has its own polyol pathway activity.