In fish, oocyte maturation (resumption of meiosis after completion of vitellogenesis and before ovulation) is triggered by maturation inducing steroids (MIS) which generally appear to be secreted in the ovary in response to stimulation by a pituitary maturational gonadotropin. Converging data from different laboratories show that 17α-hydroxy, 20β-dihydroprogesterone (17α, 20β-OH-P) is the principal MIS in salmonoids; but clear identification remains to be done in other taxonomic groups.The experiments reported here in the rainbow troutSalmo gairdneri examine the possible involvement of oocyte cAMP on the mechanism of MIS action. The action of 17α, 20β-OH-P, on germinal vesicle breakdown (GVBD) in oocytes incubatedin vitro within the follicle, was inhibited by various substances expected to elevate the intraoocyte concentrations of cAMP: cAMP (≥ 1 mM) or dibutyril cAMP (≥ 2 mM), phosphodiesterase inhibitors such as theophylline (≥ 0.2 mM) or 3-isobutyl-1 methylxanthine (IBMX ≥ 0.1 mM), adenylate cyclase activators such as cholera toxin (> 100 nM) or forskolin (≥ 0.03 mM). In fact, the combined action of IBMX (1 mM) and forskolin (0.01 or 0.05 mM)in vitro was to promote accumulation of intraoocyte cAMP within 1 to 5 hours. Oocyte cAMP concentrations exhibited a large variability between different females, depending on the stage of oocyte development; a significant positive correlation between oocyte cAMP concentration and the follicular weight, and a significant negative correlation between oocyte cAMP concentration and the median efficient dose of 17α, 20β-OH-P for induction of GVBD, were observed. Finally, when intrafollicular oocytes were incubatedin vitro, the addition of a maturation-inducing concentration of 17α, 20β-OH-P (3×10(-6)M) induced a significant decrease of oocyte cAMP within the first 10 hours of incubation. These results show that cAMP appears to play a central role in the regulation of oocyte sensitivity to 17α, 20β-OH-P and in the intraoocyte mechanisms leading to GVBD in trout.These data are discussed together with the few indications available in fish concerning the mechanism of MIS action which can be compared to some extent with the amphibian model.