Ex parte Baltimore and Ohio Railroad Company is a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue an injunction against a railroad company. The case arose when the Baltimore and Ohio Railroad Company (B&O) sought to construct a bridge over the Potomac River. The state of Maryland, however, sought to enjoin the construction of the bridge, arguing that it would interfere with navigation on the river. The Supreme Court held that the state court did not have the authority to issue the injunction. The Court reasoned that the power to regulate interstate commerce was vested in Congress, and that the state court was not authorized to interfere with the exercise of that power. The Court further held that the state court was not authorized to interfere with the construction of the bridge, as it was an interstate commerce matter. The Court's decision was significant in that it established the principle that the power to regulate interstate commerce was vested in Congress, and that the states were not authorized to interfere with the exercise of that power. This decision has been cited in numerous subsequent cases, and has been used to support the principle that Congress has the exclusive power to regulate interstate commerce.

In Ex Parte Baltimore and Ohio Railroad Company, the Supreme Court was asked to determine whether a state court had jurisdiction over a case involving an interstate railroad. The majority opinion held that the state court did not have jurisdiction because it would interfere with Congress' power to regulate interstate commerce. Justice Field dissented from this decision, arguing that while Congress has exclusive authority over matters of interstate commerce, states still retain their own powers in such cases. He argued that if there were no limits on what states could do when regulating intrastate activities related to railroads then they could effectively nullify federal laws governing these activities. Furthermore, he noted that since many of the issues involved in this case were local ones rather than national ones, it should be within the purview of state courts to decide them as long as they did not conflict with federal law or regulations.