We analyze the relation between the holding of a local mandate and parliamentary activities. Multi-mandate holding can hamper parliamentary work by reducing the time devoted to this latter activity. But it can also favor parliamentary work if activities associated with local offices allow information that is of use for parliamentary activities to be collected. We first propose a theory about this relation before testing hypotheses about parliamentary activities of French deputies during the 12th legislature from 2002 to 2007. We consider three indicators of parliamentary work: the number of notes, the number of laws and the number of questions asked to the government. We perform ordinal polynomial probit regressions of these indicators on a large set of variables including multi-mandate holding measures and the characteristics of deputies. Our main conclusion is that multi-mandate holding has a differentiated impact on the activities of deputies. Namely, holding a local mandate favors the number of questions. However, it diminishes the production of notes by deputies. Finally, we do not find clear results between holding a local mandate and the production of laws. As a result, the complementarity and the substitution effects differ between the activities of deputies.