Seasonally anoestrus Corriedale ewes receiving 1 ml GnRH (0,0042 mg buserelin-acetate) IM, were pretreated as follows: (i) 5 Days with intravaginal progesterone-impregnated sponges removed 24 h prior to GnRH (P-group); (ii) 6 h with subcutaneous oestradiol silicone rubber implants removed 6 h prior to GnRH (E-group); or (iii) no pre-GnRH treatment (0- group). The mean E₂ levels of ewes in the P-group (8,18 ± 1,52 pg/ml) and the E-group (8,03 ± 1,74 pg/ml) were elevated by 6 h prior to GnRH injection. Plasma progesterone output of GnRH-induced CL's in sheep primed with progesterone (P-group) were higher (P < 0,01) than mean values for the control ewes (O-group). The E-group was intermediate. The life-span of corpora lutea was not affected. It appears that the trophic effect of progesterone is not mediated via E₂ release, but rather as a direct effect on the ovary or hypothalamo-pituitary axis.