Through the Part D drug program, Medicare has assumed responsibility for the prescription drug needs of beneficiaries and, consequently, for an important component of mental health treatment: psychotropic drugs. Managing the prescription drug benefits of mentally ill dual eligibles could challenge this new program. Efforts to balance quality assurance and cost-effectiveness will be complicated by this population's vulnerability and high level of health care need, the uncertainties surrounding optimal psychotropic drug use, and the economic incentives of prescription-only insurers. We discuss the details and policy implications of this challenge.