This paper is concerned with the historical attempt over the last 20 years to improve integration between primary and specialist mental health care. Semi-structured interviews were carried out during the period December 2000-March 2001 with primary care workers, specialist medical and nursing staff, managers and other key informants in one large group model Health Maintenance Organization in the USA. Both overt (financial) and covert (attitudinal and conceptual) barriers to the integration of mental health and primary care were identified and the impact of these barriers on organizational development is discussed with reference to Activity Theory. The nature and quality of interprofessional conversation in an organization may be important mediating factors in addressing covert barriers to integration between primary and specialist mental health services. There may be insufficient actual contact between different groups of workers in primary and specialist care to enable these professionals to share ideas, challenge mutual assumptions and understand each others' viewpoints about the nature of their work, the covert barriers to integration. Workers may differ in the conceptual models of mental health care they utilize, their views about access to services, and the amount of information they require. In order to integrate services effectively, these issues will require discussion. Financial pressures in the system may lead to failure on the part of management to sanction and encourage opportunities for interprofessional conversation and the geographical distance between places of work may also limit opportunities for contact. However, an alternative explanation might be that attitudinal and other covert barriers to integration effectively prevent, in the first place, the development of such a shared space in which these covert barriers might actually be addressed.