Some criteria for the rational prescribing of pharmaceuticals were considered in relationship to the prescribing of a sample of British general practitioners. In particular, interest was focused on the criteria of effectiveness and safety in what is prescribed. Actual prescriptions were examined for one complete given month and the physicians writig these were judged on the criteria, using carefully specified definitions. Next the professional, educational, and demographic characteristics of the selected physicians were examined and an attempt was made to account for the manner in which physicians were judged on the two rationality criteria in terms of these charateristics. Ninety characteristics were originally considered. In the event it was shown that a reasonable discrimination of physicians' prescribing could be made on the basis of five such characteristics.