Abstract: The effect of body weight changes during the breeding season, on the ovulation rate, blood progesterone levels and embryo survival of the crossbreed Mexican ewe was studied. During may to September 2004, 120 crossbreed ewes were selected from a traditional flock in the high mountains (latitude 19� N, 99�W) of Mexico. Divided at random in four groups of 30. Group A with an average body weight of 40.2�1.7 kg was taken to pasture from 9:00 h until 18:00 h daily and received no supplement. Group B with an average weight of 43.8�2.6 kg was taken to pasture as group B and on arrival to their paddocks received 350 g/ewe of commercial concentrate. Group C (42.1�2.4 kg) was range fed only, no supplement was given. Group D (41.2�2 kg) was range fed and 15 days before the introduction of the rams flushing was given, this consisted in 300 g/ewe of a commercial concentrate. This management for all groups was initiated on the first day of May and it was continued until 15 days before the introduction of the rams (July 25). In July 10, ewes of group A showed an average weight of 41.0�2 kg, ewes of group B had a weight average of 51.2�9.1 kg. Ewes of group C and D weighed 45.3�2.3 kg and 44�1.7 kg. To induce body weight changes in groups A and B, in this day, feeding was reversed, group A were range fed and supplemented with concentrated food and ewes of group B were range fed only as previously mentioned. On august 12 weight of groups A and B was 50.3�2 and 42.6�2.5 kg (p = .001) and of groups C and D was 45.4�1.8 and 50.0�2 kg. Groups C and D continued range feeding after the flushing of group D. Body weight in September 8 was 53.7�1.7 kg; 43.5�1.9 kg; 47.8�0.9 and 52.4�2.6 for A, B, C and D groups. Teasing was continued until the end of September. Ovulation rate in ewes displaying oestrus of group A and B was 1.8�.2 and 1.3�.2 and 1.1�.2 and 1.5�.2 for groups C and D, respectively. Difference in between groups AD and BC was significant p = < 0.001. Group A produced 35 lambs. Group B 23, Group C 27 lambs and group D 29. Changes in blood progesterone were non significant, except for group B that showed higher levels. It was concluded that in order to attain higher prolificity in range fed ewes, during the following 3 to 4 weeks after mating, ewes should not suffer of nutritional distress.