In the course of screening a fetal rat cerebellum cDNA library for developmentally regulated sequences, we have identified a cDNA clone identical in sequence to that encoding a protein originally isolated from thymus, thymosin beta 10. Based on northern hybridization analyses with gene-specific oligonucleotide probes derived from the 3'-untranslated regions of thymosin beta 10 mRNA and the closely related beta 4 mRNA, we showed that both thymosin mRNAs were present at highest levels in fetal cortex and cerebellum but also were present at varying levels in all other fetal tissues examined (thymus, spleen, lung, kidney, adrenal, heart, and liver). Steady-state levels of thymosin beta 10 mRNA in cerebellum declined to negligible levels after day 14 of postnatal development. Its levels exhibited a similar pattern in developing cortex, although the adult cortex had slightly higher thymosin beta 10 mRNA levels. These results suggest that thymosin beta 10 mRNA is subject to strong developmental regulation in the rat central nervous system. Reduction of thymosin beta 10 mRNA levels also was seen during development of kidney, heart, and liver. Levels of both thymosin beta 10 and beta 4 mRNAs remained relatively constant during development of thymus, spleen, and lung. Thymosin beta 4 mRNA levels dropped much less sharply during brain development than did levels of the beta 10 mRNA. Testis and ovary contained the highest relative levels of thymosin beta 10 mRNA among adult tissues, but little thymosin beta 4 mRNA. A novel thymosin beta 10 mRNA species unique to adult testis was detected. These results indicate that both thymosins must function in the development of brain and many other organs, as well as in different subsets of organs in the adult.