The GnRH analogue [D-Ser(t-Bu)6]des-Gly10-GnRH-N-ethylamide (GnRHa, 2 μg/rat) or hCG (4 i.u./rat) was administered to hypophysectomized, PMSG-primed immature female rats. Oocyte maturation was initially detected by 2 h after GnRHa administration but the response to hCG was observed only after 4 h. Initiation of GnRHa-induced ovulation also preceded the response to hCG by 2 h. Maximal response to both these hormones was obtained at 10 and 14 h after hormone administration for oocyte maturation and ovulation respectively. The number of oocytes ovulated after GnRHa was significantly lower than that with hCG (29 ± 4 and 50 ± 7 per rat respectively; P < 0.05). Expansion of the cumulus mass and secretion of mucoid material, which are characteristic responses to LH, were also observed after GnRHa administration. However, while the action of 5 μg ovine LH/ml on the cumulus cells was mediated by cAMP, no accumulation of the nucleotide could be detected in follicles exposed to GnRHa (10-7 M). We conclude that even though GnRHa and LH/hCG seem to elicit similar responses in the ovarian follicle they differ in their kinetics, their efficiency and the mediator of their action.