With use of innovative proxies and new annual data, I demonstrate that relatively high legal capacity and regulatory activity of the early-modern Polish parliament, the Seym, was positively associated with deeper domestic commodity market integration. Conversely, the lack of effective law-making, caused by the right of a single delegate to discontinue the Seym’s sessions, fostered market fragmentation. This indicates that early parliamentary regimes required legal capacity to harmonize domestic institutions and reduce the transaction costs. The Polish case suggests a hypothesis that the pre-1800 “Little Divergence” between European parliamentary regimes could be explained by differences in their governments’ capacities.