120 Lactating Karakul ewes were randomly divided into 12 equal groups. Two main groups nursed lambs until medroxiprogesterone acetate impregnated sponges (Repromap, Upjohn) were inserted at 28, 38 and 48 days post partum (non-lactating) or until 90 days post partum (lactating). 500 or 300 I.U. PMSG (Upjohn) were injected subcutaneously at removal of the sponges. All ewes were tested for oestrus by vasectomised rams from 12 h after sponge removal. At 20 h after commencement of oestrus each ewe was laparotomized. This was repeated at 5 h intervals until ovulation occurred. Prior to each laparotomy the ewe was tested for oestrus to determine the duration of the oestrous period. The latter was highly significantly (P < 0,01) shorter in the lactating groups as well as when hormone treatment started before 48 days post partum. Oestrous activity following sponge withdrawal commenced significantly (P < 0,01) later as the post partum period increased. This resulted in significantly (P < 0,05) later ovulations relative to sponge withdrawal in the groups treated during the later post partum periods. The time of ovulation after commencement of oestrus and the number of ovulations per ewe were not influenced.