Interest in how best to influence the behaviour of clinicians in the interests of both clinical and cost effectiveness has rekindled concern with the social networks of health care professionals. Ever since the seminal work of Coleman et al. [Coleman, J.S., Katz, E., Menzel, H., 1966. Medical Innovation: A Diffusion Study. Bobbs-Merrill, Indianapolis.], networks have been seen as important in the process by which clinicians adopt (or fail to adopt) new innovations in clinical practice. Yet very little is actually known about the social networks of clinicians in modern health care settings. This paper describes the professional social networks of two groups of health care professionals, clinical directors of medicine and directors of nursing, in hospitals in England. We focus on network density, centrality and centralisation because these characteristics have been linked to access to information, social influence and social control processes. The results show that directors of nursing are more central to their networks than clinical directors of medicine and that their networks are more hierarchical. Clinical directors of medicine tend to be embedded in much more densely connected networks which we describe as cliques. The hypotheses that the networks of directors of nursing are better adapted to gathering and disseminating information than clinical directors of medicine, but that the latter could be more potent instruments for changing, or resisting changes, in clinical behaviour, follow from a number of sociological theories. We conclude that professional socialisation and structural location are important determinants of social networks and that these factors could usefully be considered in the design of strategies to inform and influence clinicians.