Blot hybridization analyses have established the presence of mRNAs for prothymosin alpha (ProT alpha) and for parathymosin (ParaT) in rat and mouse lung, liver, kidney, and brain, confirming the biosynthesis of these peptides in nonlymphoid tissues. In these tissues the levels of mRNAs paralleled the content of the polypeptides, determined with specific radioimmunoassays. The mRNA levels also confirmed the reciprocal relation between the two polypeptides; ProT alpha and its mRNA were found in highest concentrations in spleen and thymus, followed by lung, kidney, and brain, with lowest concentrations in liver. On the other hand, liver contained highest concentrations of ParaT and the mRNA for ParaT, with lowest levels present in spleen and thymus. In comparison to tissues from young (6-8 week) mice, older (18 month) mice contained lower concentrations (20-40%) of both polypeptides, with qualitatively similar decreases in mRNA content.