Pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are, in teleosts as in mammals, under the control of hypothalamic factors and steroid feedbacks. In teleosts, feedback regulations largely vary depending on species and physiological stage. In the present study the regulation of FSH and LH expression was investigated in the European eel, a fish of biological and phylogenetical interest as a representative of an early group of teleosts. The eel FSH beta subunit was cloned, sequenced and together with earlier isolated eel LH beta and glycoprotein hormone alpha (GP alpha) subunits used to study the differential regulation of LH and FSH. In situ hybridization indicated that FSH beta and LH beta are expressed by separate cells of the proximal pars distalis of the adenohypophysis, differently from the situation in mammals. The profiles of LH beta and FSH beta subunit expression were compared during experimental ovarian maturation, using dot-blot assays. Expression levels for LH beta and GP alpha increased throughout ovarian development with a positive correlation between these two subunits. Conversely, FSH beta mRNA levels decreased. To understand the role of sex steroids in these opposite variations, immature eels were treated with estradiol (E-2) and testosterone (T), both steroids being produced in eel ovaries during gonadal development. E-2 treatment induced increases in both LH beta and GP alpha mRNA levels, without any significant effect on FSH beta. In contrast, T treatment induced a decrease in FSH beta mRNA levels, without any significant effect on the other subunits. These data demonstrate that steroids exert a differential feedback on eel gonadotropin expression, with an E-2-specific positive feedback on LH and a T-specific negative feedback on FSH, leading to an opposite regulation of LH and FSH during ovarian development.