The purpose of this study was to describe the social and geographical boundaries around the networks of senior nurse executives and physician leaders and managers in acute-care hospitals in the United Kingdom. A telephone survey was conducted using standard social network methods. A random sample was drawn from a national list and repeatedly sampled until 100 respondents were interviewed. The response rate was 49.5%. Both groups tended to discuss "important professional matters" with others who were similar to themselves in terms of profession, gender, age, and seniority, with physicians being more extreme in this regard. The implication is that gaps in the network of informal ties will impede the dissemination of information and the spread of social influence between these 2 important groups. Managers (non-clinically qualified) appear to occupy a powerful "brokerage" role. Informal networks are mainly composed of local ties. The authors argue that dissemination and influence strategies that take features of the social structure into account are more likely to be successful.