In fish, like in other vertebrates, luteinizing hormone (Lh) is an essential hormone for the completion of oocyte maturation. In salmonid fish (i.e., salmon and trout), oocyte maturation is induced by Lh through its stimulation of the production of the maturation-inducing steroid, 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P). In mammals, several factors, including ovarian cytokines and growth factors, have been reported to contribute to the regulation of oocyte maturation. In fish, growing evidence suggests that tumor necrosis factor alpha (hereafter referred to as Tnf) could play multiple physiological roles in the control of ovarian function. In the present study, we have investigated the possible involvement of Tnf in the regulation of oocyte maturation in brown trout (Salmo trutta). Our results show that in vitro treatment of brown trout preovulatory follicles with coho salmon (Oncorhynchus kisutch) Lh (sLh) significantly increased oocyte maturation, as assessed by germinal vesicle breakdown (GVBD), and that this effect was blocked by TAPI-1 (an inhibitor of Tnf-converting enzyme or Tace/Adam17). Furthermore, treatment of preovulatory follicles with sLh increased the expression of tnf and tace/adam17 as well as the secretion of the Tnf protein. Importantly, recombinant trout Tnf (rtTnf) significantly increased GVBD in vitro. Our results also show that the stimulatory effects of rtTnf on oocyte maturation may be the result of the direct involvement of rtTnf in stimulating the production of the maturation-inducing steroid as evidenced, first, by the stimulatory effects of rtTnf on 17,20beta-P production in vitro and on the expression of cholesterol side-chain cleavage P450 cytochrome (p450scc) and 20beta-hydroxysteroid dehydrogenase/carbonyl reductase 1 (cbr1), the enzyme responsible for the production of 17,20beta-P, and, second, by the ability of TAPI-1 to block the stimulatory effects of sLh on 17,20beta-P production and cbr1 expression. Furthermore, sLh and rtTnf increased the expression of the Lh receptor (lhr) and decreased the expression of aromatase (cyp19a1), and TAPI-1 completely blocked the effects of sLh. These results strongly suggest that Tnf may contribute to the regulation of oocyte maturation by Lh in trout.