This thesis has addressedth e issue of primary care prescribing in different European countries. The first hypothesis investigated was: that the planned implementation of a multinational consensus-basedE uropean Formulary in primary care will result in more rational prescribing. A controlled trial involved 235 GPs from eight European countries with half the GPs participating in an educational intervention. This comprised dissemination of the Formulary and discussion of antibiotic and NSAID prescribing. Details of 101,544 doctor-patient consultations were collated and prescribing was compared and contrasted, before (Phase 1) and after the intervention (Phase 11), using performance indicators. This included measurement of the prescribing concordance with drugs recommended in the Formulary which increased by 2.9% (SEM 0.7) between Phases I to 11 in the intervention group and decreased by 1.3% (SEM 0.6) in the control group. This difference was found to be highly significant (p <0.001). Although some changes in clinical practice occurred, more notable differences were found in prescribing patterns between countries. A second hypothesis followed: that identification of the main influences on the participating GPs'prescribing will assist in the explanation of the varying effects of the Formulary in the different countries. A two-stage Delphi questionnaire study asked the GPs to identify the factors which they perceived to influence their prescribing and to rate their importance. The most important influences were drug related characteristics in six countries, followed by education/information and then patient factors. Pharmaceutical industrial factors were considered the least important influence in six countries, which followed regulatory factors in five countries. More influential factors appeared to be in the GPs' control, rather than ones imposed by national health care systems, regulation and government. The results show that the extent of Formulary adoption varied in different European countries. There remains a continuing place for the promotion of rational prescribing principally through education and information, including prescribing guidelines. Future initiatives may be more appropriate within countries but require adequate and sustained professional and government support.