The evolution of serum lipids is investigated in 26 patients admitted in a coronary care unit less than 9 hours after the onset of an acute myocardial infarction. Blood samplings are made every 3 hours until the 15th hour after the onset of the infarction. The decrease of total serum lipids concentration is significant at the 9th hour (3rd hour : 8,45 +/- 0,58 g/l ; 9th hour : 7,74 +/- 0,36 g/l) ; that of triglycerides, at the 6th hour (3rd hour : 1,33 +/- 0,17 g/l ; 6th hour : 1,18 +/- 0,21 g/l) ; that of cholesterol is more delayed, at the 9th hour (3rd hour : 2,98 +/- 0,18 g/l ; 9th hour : 2,83 +/- 0,11 g/l). The proportion of alphalipoproteins increases, that of prebetalipoproteins decreases, that of betalipoproteins remains unchanged. There is no difference between the patients who received heparin and those who did not. Total lipids and triglycerides concentrations are lower in the group of patients with complications than in the group without complications.