This study was designed to investigate platelet aggregation, plasma thromboxane A, and prostacyclin concentration and platelet sensitivity to prostacyclin simultaneously during the first month after myocardial infarction (MI). Spontaneous platelet aggregation and aggregation responses to ADP and adrenaline were low on the day of admission, increased rapidly by the 7th day post-MI, remained elevated during the second week post-MI and reached the level of chronic coronary artery disease patients but not healthy persons at the end of the fourth week of illness. An increase in plasma thromboxane B, the spontaneous and stable breakdown product of thromboxane A, level and enhanced prostacyclin production, with a maximum on the third post-MI day, were observed. We also demonstrated a significant platelet resistance to prostacyclin in MI patients. Thrombocyte sensitivity to prostacyclin normalized by the end of the fourth post-MI week. These results indicate the need for therapy with platelet inhibitors in patients with MI.