Abstract: The present research evaluated the inta-and postdialytic changes in PF4 and βTG plasma levels by ELISA method and platelet aggregation by ADP as well as flow cytometric percentage of annexin V-positive platelets as a measure of PS externalization and ultrastructural examination of platelets in 37 uremic patients on regular hemodialysis and 25 age and sex matched controls. PF4 plasma levels increased, remain consistently high during hemodialysis session (20.24±3.05 IU mL-1 after 30 min, p<0.001 and 23.67±3.68 IU mL-1 after 240 min, p<0.001) and returned to control values (6.10±1.54 IU mL-1) only after 24 h following the end of the session. βTG showed a similar trend to PF4. Platelet aggregation by ADP showed reduced function in comparison to controls (69.32±12.37% versus 91.95±1.59%, p<0.001). Flow cytometric percentage of annexin V-positive platelet, was significantly elevated (p<0.001) in uremic patients when compared to normal controls. Ultrastructural studies of platelets 30 min after starting of dialysis showing degranulation of its granules and at 240 min showing complete degranulation, while in the postdialytic phase (12 h after the end of dialysis) refilled α granules started to appear. Positive correlations were found between platelet concentration and both PF4 and βTG plasma levels during and after dialysis (p<0.001). Positive correlations between PF4 and βTG plasma levels during and after dialysis (p<0.001) and annexin V-positive platelets percentage were positively correlated with platelet concentration and both PF4 and βTG plasma levels during and after dialysis (p<0.001). Conclusion, activated platelets were found in chronic hemodialysis patients, a finding that may explain why uremics often suffer from thrombotic accidents. The platelet activation is associated with exposure of PS on the platelet exterior. PF4 and βTG are released from platelets mainly as consequence of the blood-membrane contact during dialysis and they return only slowly to control values. Understanding of the mechanisms of platelet activation may be critical in limiting the severity of thromboembolic events in uremic patients.