PROSPECT (Prevention of Suicide in Primary care Elderly-Collaborative Trial) is testing whether a trained clinician (the 'health specialist') can work in close collaboration with a primary care physician to implement a comprehensive depression management program and improve outcomes in older depressed patients. An algorithm guiding the selection and use of antidepressant medications has been developed to assist PROSPECT health specialists. This algorithm is presented and the rationale underlying the proposed treatment sequence is discussed. The PROSPECT algorithm builds upon existing guidelines after updating them and adapting them to the special circumstances of older primary care patients. Special attention has been paid to the tolerability and the target doses of the recommended antidepressant agents and to the duration of antidepressant trials. Patients who are unable to tolerate or do not respond to an antidepressant can be switched to another agent or be treated with interpersonal psychotherapy. Agents that produce only a partial response can be combined with other antidepressants or with interpersonal psychotherapy. Treatments for which empirical evidence exists are favored. However, treatments that are often poorly tolerated by elderly patients are given lower priority than treatments more likely to be tolerated. Similarly, trials that are simpler to implement in primary care are favored.