Cloning of cDNA coding for rat phosphoribosyl pyrophosphate (PPRibP) synthetase (EC 18.104.22.168) revealed two distinct types of subunit, referred to as PRS I and PRS II (Taira et al. (1987) J. Biol. Chem. 262, 14867-14870). Tissue-specific expression of PRS I and PRS II genes (designated PRPS1 and PRPS2, respectively), was shown for 16 rat organs, using Northern blot analysis. The 2.3 kb PRPS1 mRNA level was high in the brain and adrenal gland, whereas the 3.7 kb PRPS2 mRNA level prevailed in the lung and spleen. Both genes were highly expressed in the thymus, adipose tissue and testis. In other mammals (mouse, calf and human), these two types of mRNA were also detected in various tissues and cell lines. Thus, the expression of each gene is regulated in a tissue-specific manner and there may be functional differences between catalytic and/or regulatory properties of subunits PRS I and II of this enzyme. In the testis, an additional PRPS1-related transcript of 1.4 kb was noted in rats, mice and humans. This transcript may belong to a group of testis-specific gene expressions or functions.