Thrombokinetic studies were carried out on 26 consecutive patients with myocardial infarction (MI) admitted to a coronary care unit in the acute stage during a two-month period. The results were compared with those of an age-matched control group. In the MI patients, platelet mean life-span was 5.0 +/- 0.3 days and significantly shorter (p greater than 0.01) than in the controls (6.4 +/- 0.4 days). The mean platelet production rate for the patients with MI was significantly higher (p greater than 0.005) than for the controls. On the basis of results reported by others as well as the present data, it is suggested that during the acute phase of MI there is no additional measurable reduction in platelet survival above that observed in chronic coronary artery disease.