Restriction fragment length polymorphisms were identified in sheep and deer using ovine cDNA probes for the FSH receptor (FSHR) and the LH receptor (LHCGR). FSHR and LHCGR were closely linked in sheep with no recombinants and neither receptor was linked to the Booroola fecundity gene (FecB). Both receptors were also closely linked in deer at a map distance of 3·3 cM. Linkage between the receptor genes assigns FSHR to sheep chromosome 3. Sequence analysis showed that the mammalian LHCGRs and FSHRs are more similar to each other than to mammalian TSH receptor (TSHR). Taken together, these data suggest that TSHR and the LHCGR/FSHR arose from a common ancestral gene by a process of chromosomal duplication. Subsequent duplication of the region containing the LH/FSH receptor and functional divergence could have given rise to the two gonadotrophin receptors present in mammals today.