The rat c-erbA? cDNA fragment was used as probe to investigate the expression of T3 receptor gene in brain, heart and kidney in hyperthyroid and hypothyroid rats. There were three types of c-erbA? mRNA-6. 0kb, -5. 0kb and -2. 0kb in all examined tissues. In heart and kidney, the levels of c-erbA? mRNA decreased in hyperthyroid rats and increased in hypothyroid rats. It might be a compensatory mechanism. In brain, however, the level of c-erbA? mRNA was unaffected by thyroid status. The results suggest that the T3 receptor gene may have an auto-regulatory mechanism in rat heart and kidney under different thyroid status.