Fundamental changes in the delivery of primary medical care outside normal surgery hours are under consideration in Great Britain.Published research into the provision and utilisation of out of hours services shows long term trends towards decreasing personal commitment among general practitioners and rising demand from patients for primary and hospital accident and emergency department care. Wide variations exist regionally, locally, and between practices. Previous studies, however, have been limited in scope and provide an inadequate basis for assessing the potential impact of change. The overall demand for care across all sources of provision cannot be measured: there is a lack of data on costs, and evaluative studies comparing alternative patterns of service delivery have rarely been undertaken. A period of experimentation and evaluation of a range of options should precede the wider adoption of any particular models.