Mo. Const. art. III, § 6. Qualifications of senators

Each senator shall be thirty years of age, and next before the day of his election shall have been a qualified voter of the state for three years and a resident of the district which he is chosen to represent for one year, if such district shall have been so long established, and if not, then of the district or districts from which the same shall have been taken. [Effective - 27 Feb 1945 , see footnote]