A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the pituitary of the European sea bass, Dicentrarchus labrax. The complete cDNA is 1814 base pairs (bp) in length and encodes a protein of 416 amino acids. The 5′ UTR and 3′ UTR are 239 bp and 324 bp in size, respectively. The expression sites of this GnRH-R were studied in the brain and pituitary of sea bass by means of in situ hybridization. A quantitative analysis of the expression of the GnRH-R gene along the reproductive cycle was also performed. The GnRH-R brain expression was especially relevant in the ventral telencephalon and rostral preoptic area. Some GnRH-R messenger-expressing cells were also evident in the dorsal telencephalon, caudal preoptic area, ventral thalamus, and periventricular hypothalamus. A conspicuous and specific GnRH-R expression was detected in the pineal gland. The highest expression of the GnRH-R gene was observed in the proximal pars distalis of the pituitary. This expression was evident in all LH cells and some FSH cells but not in somatotrophs. In the pituitary, the quantitative analysis revealed a higher expression of GnRH-R gene during late vitellogenesis in comparison with maturation, spawning, and postspawning/resting periods. However, in the brain, the highest GnRH-R expression was evident at spawning or postspawning/ resting periods. These results suggest that the expression of this GnRH-R is regulated in a different manner in the brain and the pituitary of sea bass.