This research investigates the linkage between political change and legislative institutional change in the Mexican Chamber of Deputies since 1970. A series of electoral reforms beginning in 1977 allowed the opposition parties to increase their Chamber representation. How were issues of internal organization resolved in the new political context of strengthened opposition conflict? How did these institutional changes affect the Chamber activity? In contrast to the efficiency and specialization perspective from Organization Theory, the theoretical approach to these questions directs attention to how members try to shape the legislative structure and rules to advantage themselves. In this research, I track the institutional and behavioral changes in the Chamber floor and committee system since 1970 with an interrupted time series design. These political and institutional changes reflect the power and strategic interaction between the opposition and ruling majority party, PRI (Institutional Revolutionary Party). The majority party restricted the institutional rules and structures in the early post-reform legislatures until the opposition increased to an average of 41% of the seats in the 1988-97 legislatures. Then, the opposition parties were able to force some institutional changes to allow them more political space. Currently, the opposition holds a majority in the Chamber and has capitalized on the gradual changes that have been taking place since the late 1970s.