Acute myocardial ischemia was produced by ligature of the anterior descending coronary artery on 658 dogs in 3 separate laboratories. Overall, 12% of the dogs died within the first hour (instantaneous death) and 25% within the first 24 hours (sudden death). The sudden death rate was significantly related to the logarithm of the weight of the dogs in the range studied. It varied widely if values from small series, comprising the same number of dogs, were considered. Values became less dispersed as the size of the series increased. In series of 10 dogs, sudden death rates ranged from 0 to 70%, whereas in series of 100 dogs the range was 14 to 36%. Stable values were obtained for 50 to 60 dogs per series. Accordingly, reliable assessment of preventive measures can be made only with experimental series of at least this size.