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Specific receptors for gonadotrophin-releasing hormone (GnRH) were extracted from the rat pituitary gland with several detergents and characterized by binding studies with the potent GnRH antagonist [Ac-D-pCl-Phe^1,2, D-Trp³, D-Lys⁶, D-Ala¹⁰]-GnRH (GnRHant). The particulate GnRH receptors were most effectively solubilized with 5 mM 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulphonate (CHAPS), which extracted 63% of the original membrane binding activity when assayed with ¹²⁵I-labelled GnRHant. The binding affinities of particulate and CHAPS-solubilized receptors analysed with ¹²⁵I-labelled GnRHant were 1·5 ± 0·4 x 10⁹ M^–1 and 1·2 ± 0·2 x 10⁹ M^–1 respectively. Gel filtration of the CHAPS-solubilized receptor revealed a major peak of specific binding activity with M_r of about 700 000. A hormone—receptor complex of similar M_r was observed when CHAPS-solubilized receptors were labelled with photoreactive radioiodinated [D-Lys⁶]-des-Gly¹⁰-GnRH-N-ethylamide and then analysed by gel chromatography. However, when pituitary particles were photolabelled and solubilized in 2% Triton X-100 before analysis on Sephacryl S-300, the M_r of the receptor was approximately 250 000, similar to the value obtained by sucrose density gradient centrifugation of the CHAPS-solubilized receptor. After solubilization in sodium dodecyl sulphate (SDS) the photolabelled receptor was eluted from Sephacryl S-300 as a 60 kDa peak which on SDS-gel electrophoresis contained a 52 kDa component, corresponding to the major binding subunit extracted directly from photolabelled pituitary membranes. The difference in higher molecular weight forms observed under non-denaturing and denaturing conditions could reflect the need for additional membrane components to maintain the active conformation of the GnRH receptor site. Whereas the minimum M_r of the solubilized receptor is about 250 000 under non-denaturing conditions, analysis of the photolabelled GnRH-receptor complex by SDS chromatography and electrophoresis indicates that a binding subunit with M_r of 50 000–60 000 is present in the GnRH holoreceptor.