Summary. In Exp. 1, ovulation rate was measured in three groups of Romanov ewes given two injections of 600 i.u. PMSG 3 weeks apart with the ewes intact (Group I, N = 8), a similar treatment with the ewes intact at the first injection and unilaterally ovariectomized at the second (Group II, N = 8), or unstimulated ewes which were hemispayed at the same time as Group II ewes (Group III, N = 6).
In Exp. 2, the follicular population of one ovary was correlated with the number of ovulations induced by 600 i.u. PMSG in the contralateral ovary (10 Romanov ewes).
From 8·4 ± 1·8 (Group I) and 8·2 ± 3·3 (Group II) CL at the first injection, PMSG-induced ovulation rate at the second injection decreased to 3·9 ± 1·8 and 3·7 ± 1·2 in Groups I and II respectively, a value similar for ewes with 1 or 2 ovaries. Furthermore, despite no major changes in the number of antral follicles after the first injection, there was no correlation (r = −0·09) between the response to the two successive injections in intact ewes.
Comparison of the ovarian status of the ovary removed before the PMSG injection (Group II ewes of Exp. 1, ewes of Exp. 2) to the number of CL found in the remaining ovary demonstrated that PMSG-induced ovulation rate was (i) not correlated with the overall antral follicle population (r = 0·62 in Exp. 1, r = 0·49 in Exp. 2), (ii) significantly correlated (r = 0·74, P < 0·05, in Exp. 1; r = 0·85, P < 0·01, in Exp. 2) with the number of healthy follicles 0·8–2·0 mm in diameter, and (iii) negatively correlated with the number of healthy follicles >2 mm in diameter (r = −0·29 in Exp. 1;r = −0·61, P < 0·05 in Exp. 2).