This paper examines the role of the police in supporting community based mental health services for the chronically mentally ill in a mid-sized midwestern city. Cooperation between the police and mental health system, as reflected in training and procedural agreements for emergency evaluations and hospitalization, achieves the conversion of the police to a medical model view of mental illness which stresses the importance and effectiveness of medication compliance so that the police will act upon this belief in their handling of the mentally ill. However, police recognition of the limits of the medical model and organizational change impacts upon cooperative police practices. Police experience conflict over the assumed therapeutic goals of community treatment and the important gatekeeping function community mental health systems perform under deinstitutionalization. Erosion of the police belief in the effectiveness of community mental health, and the resulting change in practices, may provide an important, yet partial, counterbalance to the control exerted by the medical model. The availability of police authority to assure compliance, or in some cases to reinforce the consequences of non-compliance with medication, raises questions about police-mental health relationships that are too cooperative.