Among the Uralic languages, Mordvin (with two standard languages, Moksha and Erzya) possesses a uniquely complete and complicated definite conjugation. In addition to the subject, the direct object (principally: in all three persons, singular and plural) can be expressed morphologically. The Mordvin definite conjugation is also unique in the western branches of Uralic; thus, its history and relationship with the definite conjugation of the eastern branches (Ugric, including Hungarian, and Samoyed) is an intriguing question. This study is based on Prof. Keresztes's earlier studies on Mordvin historical phonology and on empirical material, also collected on recent field trips.