An HPLC method was used to tentatively identify progesterone (P₄) and its metabolites (17-hydroxyprogesterone (17-P₄) and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P)), corticosteroids (cortisol and corticosterone) and testosterone in ovary/follicular preparations of the catfish Heteropneustes fossilis associated with in vivo or in vitro oocyte maturation/ovulation. A single i.p. injection of human chorionic gonadotrophin (100 IU/fish, sampled at 0, 8 and 16 h) induced oocyte maturation and ovulation, which coincided with significant and progressive increases in 17,20β-P, and P₄ and 17-P₄, the precursors of the former. Both cortisol and corticosterone also increased significantly. Conversely, testosterone decreased significantly and progressively over time. Under in vitro conditions, incubation of post-vitellogenic (intact) follicles or follicular envelope (layer) with 2-hydroxyoestradiol (2-OHE₂, 5 μM for 0, 6 and 24 h) elicited a sharp significant increase in 17,20β-P, the increase being higher in the follicular envelope incubate. P₄ and 17-P₄ also registered significant increases over the time with the peak values at 24 h. Cortisol and corticosterone increased significantly in the intact follicle, but not in the follicular envelope incubate. Testosterone decreased significantly in the intact follicle, but increased significantly (24 h) in the follicular envelope incubate. Coincident with these changes, the percentage of germinal vesicle breakdown (GVBD) increased over the time in the intact follicle incubate (48.9% at 6 h and 79.8% at 24 h). Denuded oocytes on incubation with 2-OHE₂ (5 μM) did not produce any significant change in the percentage of GVBD or in the steroid profile. While corticosterone and 17,20β-P were undetected, P₄, 17-P₄, cortisol and testosterone were detected in low amounts. The results show that the 2-OHE₂-induced GVBD response seems to be mediated through the production of 17,20β-P and corticosteroids. It is suggested that hydroxyoestrogens seem to be a component in the gonadotrophin cascade of regulation of oocyte maturation/ovulation in the catfish.