In Project Nord-Gudbrandsdal we aimed to reduce prehospital mortality from myocardial infarction in a sparsely populated area. The ambulance system consisted of local ambulances with semi-automatic defibrillators (the ambulance personnel were trained in the use of these), combined with advanced medical support from the local doctor and/or the air ambulance doctor. During the three-year period (1987-89) 75 prehospital cardiac arrests were observed in a population of 22,000 inhabitants. 12 patients with ventricular fibrillation were defibrillated, and eight primarily with success. Four patients were discharged from hospital in a satisfactory condition. We discuss the factors determining successful resuscitation.