Icelandic sheep, the only breed in Iceland, belongs to the North European short-tailed group of sheep which includes the Finnish Landrace and Swedish Landrace breeds (Adalsteinsson, 1970). The breed was brought to Iceland during the settlement period (870-930 AD) and it is believed that there has been little if any genetic introduction from outside the country during the last 200 years. The average litter size is about 1.7 with a remarkably low incidnce (˂1.5%) of triplets. The conception rate to a single cycle in the absence of synchronisation is reputedly greater than 90%. There is also evidence for the presence of a gene (Thoka gene) with a large effect on litter size in certain flocks (Jonmundsson and Adalsteinsson, 1985). The low incidence of triplet births and the apparently high conception rate suggest that the distribtuion of ovulation rate and the level of embryo survival may be different from those of typical British breeds.