Nucleotide sequence comparisons of the single gene for the human luteinizing hormone gene beta subunit with two of the seven genes for the human chorionic gonadotropin beta subunit suggest that the beta human chorionic gonadotropin genes have evolved from an ancestral beta luteinizing hormone gene by a series of selected changes with very little neutral drift. Moreover, the 24 amino acid carboxy-terminal extension of the human chorionic gonadotropin beta subunit appears to have arisen by a single base deletion that incorporated the 3'-untranslated region of the ancestral beta luteinizing hormone gene into the coding region.