The necessity to calculate nursing needs in terms of staff numbers is recognized as a longstanding one, and attempts to do this scientifically are not new. It is noted that the majority of studies have been carried out in the acute nursing field and the model used has invariably been a medical one, by orientation if not by design. Some of the problems and longterm consequences of former approaches are discussed. Lack of suitable measures in mental handicap and mental illness have led to the imposition of arbitrary staffing levels. To make any meaningful assessment of nursing needs in these fields it is suggested that an alternative to a medical model should be used. It is proposed that a behavioural model is the most logical choice. Comparisons between the medical and behavioural models are made. The application of a behavioural framework to the problem is described, and the advantages of the model in terms of practical application, the generation of quantifiable data and the ease with which the problem can be operationalized, are discussed. Methods of assessing outcome of nursing practice in relation to the institutional function (as defined) are described. The appropriateness of using nurses in the proposed role is considered as are the implications for nurse training.