Numerous studies have shown that in the acyclic ewe, the fIrst corpus luteum (CL) after administering gonadotropin releasing hormone (GnRH) will lead to inadequate progesterone (P4) concentrations or will be short-lived. An attempt was made to promote the function of the GnRH-induced CL in lactating ewes (n = 70) either by increasing endogenous LH release through administration of naloxone (4 injections at 1 mg/kg body mass every 2 hrs over 3 days) or via an exogenous supply of gonadotropin (PMSG, 100 I.U., twice daily for 3 days). Treatments were applied in an effort to promote maturation of the preovulatory follicle or to stimulate the CL. Naloxone, either before or after, and PMSG prior to GnRH (Day 0) did not increase plasma P4 concentrations. PMSG on Days 3-5 after GnRH was more effective in promoting luteal function (compared to untreated controls) than similar treatment on Days 6-8 or 9-11. This was evidenced by a greater (P <0,05) total area under the P4 curve (34,2 ± 2,9 vs. 25,7 ± 2,9 units), an increased (P< 0,05) number of days for which P4 concentrations exceeded 2,0 ng/ml (6,6 ± 1,5 vs. 2,5 ± 1,1 days) and a higher (P <0,05) P4 level on Days 8-12. Only the P4 concentrations on Days 10 and 11 were improved (P <0,05) by PMSG on Days 6-8, while PMSG on Days 9-11 was without significant effect. Luteotropic support of the CL for a short period prior to mid-cycle was thus more beneficial to luteal function than stimulation at any other stage.