Graves' ophthalmopathy occurs in up to 90% of patients with Graves' disease, supporting the notion of a common denominator in the development of these two disorders. The thyrotropin receptor has been proposed as the link for this clinical association. In the present study we have investigated whether thyrotropin receptor mRNA species exist in extraocular muscle and non-ocular skeletal muscle by reverse transcription-polymerase chain reaction (RT-PCR). We have, with high stringency RT-PCR, Southern analysis, and direct sequencing of PCR products, identified for the first time the presence of both full length and splice variant thyrotropin receptor mRNA in extraocular but not non-ocular skeletal muscle. This extraocular muscle thyrotropin receptor expression was shared, as expected, with normal thyroid but not other control tissues including brain and kidney. These data demonstrate that the thyrotropin receptor, the autoimmune target of Graves' disease, is exclusively expressed in extraocular muscle as well as the thyroid and lend support to the notion that it is a likely candidate autoantigen in Graves' ophthalmopathy.