This research note contributes to the comparative studies on legislative careers. It sheds light on the scarcely researched members of four Latin American upper houses, the Senates of Argentina, Brazil, Chile, and Uruguay. It examines both the basic social features of the parliamentary elite—age, gender, level of education, and university careers—and the legislators' political careers. The goal is to find out whether the upper houses are composed of a larger proportion of senior members than the respective lower chambers, that is, of members who not only are older, wealthier, and more educated, but that also have greater political experience. As a result of this research, the Senates of Chile and Uruguay stood out for having the largest share of senior members. The Brazilian Senate followed them with a considerable level of seniority. As a consequence of a series of institutional reforms based on the 1994 constitution, the Argentine Senate differed from the other three cases as being a much junior chamber.