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The description of two or more forms of gonadotropin‐releasing hormone (GnRH) in most vertebrates suggests multiple roles for this family of peptide hormones. In order to verify these functions, we analysed the anatomical location, time of initial expression and ontogenic changes in three distinct GnRH receptors (GnRH‐Rs) in developing and sexually mature tilapia, using antisera raised against the extracellular loop three of the receptor, which is a determinant in ligand‐selectivity and receptor coupling to signalling pathways. In all age groups, including males and females, using in situ hybridization and double‐label immunological methods, GnRH‐R type IA was colocalized in cells containing luteinizing hormone (LH) β‐subunit in the pituitary. GnRH‐R type IB was visualized in prolactin cells and LH cells. The type III GnRH‐R was expressed in growth hormone cells. On day 8 after fertilization, GnRH‐R type III …