This study has as its central focus an investigation into the existence and nature of the apprenticeship norm in the United States Senate. Over its history, the Senate has been frequently portrayed as a body guided by rather restrictive, informal rules of behavior for its members. The apprenticeship norm has been identified by some as the most important of these rules; contributing to the Senate's centralized and conservative policy orientation. More recently, however, it has been argued that the Senate has become a more decentralized and fragmented body within which the apprenticeship norm is no longer important. The present study offers for the first time an empirical test of the existence and nature of the apprenticeship norm for selected sessions of the Senate for the time period 1940-1976. The frequency of performance of various types of floor activity by members of the Senate were correlated and regressed with years of service in the Senate as well as with other background characteristics of Senators to test both for the existence of the apprenticeship norm as well as to identify its relevance relative to other potential explanations of Senate floor behavior. Several definitions of apprenticeship were advanced and tested.