We have studied the incidence of possible triggers of the myocardial infarction regarding its site in 750 patients with anterior and 731 patients with inferior infarction. Infarctions occurred most frequently without recalling any triggering activity, especially in patients with anterior infarction (67 vs. 44%). Physical effort as the possible precipitator was also more frequent in anterior infarctions (22 vs. 16%). However, the onset of inferior infarction was more frequent during meteorological stress (9 vs. 2%), emotional stress (10 vs. 3%), after overeating (13 vs. 3%) and nicotine abuse (6 vs. 1.5%). These triggers were independent and highly significant (P < 0.02 in each case) discriminators of the site of myocardial infarction. Bimodal circadian rhythm, with primary peak between 6 and 9 h a.m. and the secondary peak between 3 and 6 p.m. was observed in patients which did not recall any triggering activity, and this was more pronounced in patients with inferior infarction. These results support the hypothesis that the influence of the vegetative tone is most pronounced in the onset of myocardial infarction of inferior wall.