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.