Concern that donations to political campaigns secure preferential treatment from policy makers has long occupied judges, scholars, and the public. However, the effects of contributions on policy makers' behavior are notoriously difficult to assess. We present the first randomized field experiment on the topic. The experiment focuses on whether contributions facilitate access to influential policy makers. In the experiment, a political organization attempted to schedule meetings between 191 congressional offices and the organization's members in their districts who were campaign donors. However, the organization randomly assigned whether it revealed to congressional offices that prospective attendees had contributed to campaigns. When informed prospective attendees were political donors, senior policy makers made themselves available between three and four times more often. These findings underscore concerns about the Supreme Court's recent decisions deregulating campaign finance.