The insulin receptor (IR) is one participant in the partitioning of absorbed nutrients to, and between, the insulin-responsive tissues of animals. We previously reported the alternative splicing of a 36 base pair exon 11 segment of the sheep IR gene and found differences in the ratio of the resultant 11 m-RNA and 11+m-RNA isoforms between muscle and fat depots in crossbred, but not purebred, Texel lambs (McGrattan et al., 1998). Because the IR isoforms, when expressed in cultured cell lines, differ in affinity for insulin by two fold or more (McClain, 1991), variations in the ratio of IR isoform expression may have practical significance for the partitioning of nutrients between tissues and thus for differences in lean and fat gain between the purebred and crossbred lambs. Similarly, variation in IR m-RNA isoform ratios amongst the adipose depots in different breeds of dairy cow could have practical significance for differences in the ability of the breeds to store and mobilise energy and thus for differences in lactational performance between dairy breeds. This study is a preliminary investigation of this hypothesis.