A cDNA sequence from the gonadotrophin alpha-subunit mRNA of Australian brushtail possum (Trichosurus vulpecula) has been determined and analysed. Comparison with seven eutherian mammalian gonadotrophin alpha-subunit gene sequences revealed an average of 82.6% homology between the coding region nucleotide sequences and 88.8% identity between the predicted amino acid sequences. The predicted possum gonadotrophin alpha-subunit protein has ten evolutionarily conserved cysteine residues, two potential N-linked glycosylation sites and a putative enzyme recognition sequence which it has been suggested is required for sulphation of carbohydrate moieties. Comparison of the possum gonadotrophin alpha-subunit 3' untranslated region (UTR) sequence with the 3' UTRs of eutherian alpha-subunit transcripts revealed sequence homology. In particular, an 18 nucleotide imperfect palindromic sequence present in the possum 3' UTR, with the potential to form a hairpin loop, was found to be evolutionarily conserved and present in five out of seven eutherian alpha-subunit 3' UTR sequences. In situ hybridization localized the transcripts to a sub-population of anterior pituitary cells presumed to be gonadotrophs and thyrotrophs. In summary, these results indicate considerable conservation of the structure and function of the gonadotrophin alpha-subunit protein since the divergence of the marsupial and eutherian mammalian lineages.