Gonadotropin-releasing hormone (GnRH) plays a pivotal role in the regulation of reproduction in vertebrates through interaction with a specific receptor. The GnRH-stimulated gonadotropin synthesis and release are regulated by the GnRH receptors (GnRHRs). In this study, we have identified a GnRH receptor (GnRHR) gene from the hermaphroditic fish Kryptolebias marmoratus. K. marmoratus GnRHR showed typical vertebrate GnRHR domains and motifs, and its cDNA contained 1634 bp including an open reading frame (ORF) of 1263 bp encoding a putative protein of 420 amino acids. To analyze expression patterns of GnRHR gene in various tissues and developmental stages of K. marmoratus, we carried out quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR). The K. marmoratus GnRHR gene expression was detected in all the tissues of adult fish with highest level in brain and gonad. The expression of K. marmoratus GnRHR mRNA increased from stage 1 (2 day post fertilization, dpf) to stage 4 (12 dpf) but steeply decreased at hatching stage (stage 5). Expression of K. marmoratus GnRHR after exposure to endocrine-disrupting chemicals such bisphenol A (BPA, 600 microg/L for 96 h) and 4-tert-octylphenol (OP, 300 microg/L for 96 h) in hermaphrodites as well as secondary males was highly up-regulated in almost all the tissues. Another EDC, 4-nonylphenol (NP, 300 microg/L for 96 h) showed no consistent response. 17beta-estrodiol (E2, 100 ng/L for 96 h), a known natural estrogen, suppressed expression of GnRHR in most of the tissues from hermaphrodites as well as secondary males. Tamoxifen (TMX, 10 microg/L), an estrogen antagonist, on the other hand, caused upregulation of GnRHR expression in the liver of hermaphrodites and the gonad and liver of secondary males. This is the first report of a GnRHR gene from K. marmoratus and modulation of its expression by EDCs. This study provides an insight into the molecular mechanism of endocrinological functions of this unique fish.