Three cellular homologs of the v-erbA oncogene were previously identified in the rat; two of them encode high affinity receptors for the thyroid hormone triiodothyronine (T₃). A rat complementary DNA clone encoding a T₃ receptor form of the ErbA protein, called r-ErbAβ-2, was isolated. The r-ErbAβ-2 protein differs at its amino terminus from the previously described rat protein encoded by c-erbAβ and referred to as r-ErbAβ-1. Unlike the other members of the c-erbA proto-oncogene family, which have a wide tissue distribution, r-erbAβ-2 appears to be expressed only in the anterior pituitary gland. In addition, thyroid hormone downregulates r-erbAβ-2 messenger RNA but not r-erbAβ-1 messenger RNA in a pituitary tumor-derived cell line. The presence of a pituitary-specific form of the thyroid hormone receptor that may be selectively regulated by thyroid hormone could be important for the differential regulation of gene expression by T₃ in the pituitary gland.