The aim of this study was to elucidate the putative role of the gonadotropin releasing hormone (GnRH or LHRH) as an olfactory stimulant in fish. We report for the first time extreme sensitivity of the olfactory organ in the rainbow trout (Oncorhynchus mykiss) to GnRH. Recordings of the electroolfactogram (EOG) showed an electrophysiological response to 10(-16) M GnRH four times the amplitude of the response to a fresh water control stimulus. By stimulating the olfactory epithelium with several GnRH analogs and fragments of the decapeptide, the biologically active region of GnRH could be partly elucidated. The response profile of GnRH differed from that of the positive control odorant L-alanine, suggesting that separate receptors or receptor cells are involved. We propose that this potent odorant may act as a reproductive pheromone in fish.