During December mating periods of 2000-2003, wool and meat breeds crossed with East Friesian milk sheep and rams of prolific breeds (Finn or Romanov) were examined laparoscopically. Ovine ovaries were examined between 4 and 9 days after mating. The litter size was recorded and compared with the number of ovulations. The fertility of the analyzed animals was 69.6%, with 192.6% prolificacy. The difference between the average number of corpora lutea found on ovaries and the mean number of lambs born was 0.35 and ranged from 0.32 to 0.41 according to age group, from 0.22 to 0.41 according to sheep birth type, and from 0 to 0.52 lamb per ewe according to percentage of prolific breeds.