Scholars have documented long-term declines in mainstream media trust in the United States and elsewhere, a trend that raises concerns about the ability of citizens to hold their governments to account amid increasing polarization. Underlying these concerns are basic questions about the relationship between people's trust in news sources and the extent to which their attitudes are subject to change when they encounter new information from those sources. Early studies proposed the core insight that persuasion hinges on source credibility: how much people find information from a source to be believable. However, existing research on the topic almost universally takes source credibility as given. This is problematic because patterns of trust can be confounded in any number of ways. To overcome this problem, we create a hypothetical news outlet and manipulate participants' perceived credibility in this source. We then randomly assign subjects to read op-eds on topics attributed to the source's editorial board. Our three-wave design allows us to test the persistence of both our credibility manipulation and the persuasive effect of exposure to information from our news source. In doing so, we provide for the first time a comprehensive test of classical expectations that information from trusted intermediaries is more persuasive. Promisingly, our credibility treatments produce lasting effects on trust in our mock news source. We find some evidence that greater source credibility boosts the persuasiveness of arguments about a relatively unpoliticized topic, though this appears not to be the case for two issues that more clearly divide partisans.