In an attempt to understand the molecular mechanisms by which steroids can modulate brain functions in fish, we first localized the cells which produce estrogen receptor mRNA in the rainbow trout forebrain (Salbert et al., 1991). We now report how estradiol itself can alter the estrogen receptor mRNA content of these cells in a sterile strain of female rainbow trout. We also examined liver and pituitary levels of the estrogen receptor mRNA under the same estrogenic treatment. As revealed by slot blot and in situ hybridisations, a single injection (1.5 mg/kg) of estradiol can induce a strong increase (about five-fold) in the estrogen receptor mRNA levels in the liver, as well as a moderate increase (about two-fold) in two nuclei of the hypothalamus/preoptic area: the nucleus lateralis tuberis and the nucleus preopticus periventricularis. Conversely, no modifications of these levels were observed in the pars intermedia and the proximal pars distalis of the pituitary. Moreover, a comparison between estrogen receptor mRNA levels in the brain of sexually active female trout and in the brain of these sterile animals revealed that, in these latter, estrogen receptor mRNA levels are lower but can be increased by a single estradiol injection and reached the levels observed in mature females.