Platelet function and platelet sensitivity to PGI2 was studied before and after jogging in seven males and four females in good health. The ADP-induced (1 microM) aggregation, being always increased after physical activity, showed during the entire follow-up period of 12 months a significant decrease at rest and after exercise compared with baseline values. The change in the aggregation response was significant for t alpha at month 1 and for delta tmax at month 2 after starting the jogging. The platelet sensitivity to PGI2, being always diminished after exercise, also increased significantly during the entire follow-up period compared with baseline levels. In contrast to the ADP-induced changes, the changes in platelet sensitivity were significant already at week 2. Our findings indicate that daily physical activity causes a significant improvement in platelet function and reduces platelet activation.