Democratic theory must pay attention to what influences public opinion. In this study the content of network television news is shown to account for a high proportion of aggregate changes (from one survey to another) in U.S. citizens' policy preferences. Different news sources have different effects. News commentators (perhaps reflecting elite or national consensus or media biases) have a very strong positive impact, as do experts. Popular presidents tend to have positive effects, while unpopular presidents do not. In contrast, special interest groups tend to have a negative impact.