A group supporting the marriage amendment says it is putting up about 25 billboards on major roads in the Twin Cities, St. Cloud, Duluth and Rochester.
Minnesota for Marriage says one of them is in St. Paul, visible from eastbound I-94, near Cleveland Avenue.
On the billboards are pictures of couples and families with the words "Marriage = One Man + One Woman" and "Vote YES."
The group is supporting a proposed constitutional amendment that would define marriage as only between one man and one woman. Gay marriage is already illegal in the state, but supporters want it put into the constitution to make it harder for the courts or legislators to change.
The pro-amendment group says it will unveil other forms of advertising in the weeks leading up to the Nov. 6 election.