Objective The effect of the emulsion in which propofol is formulated on blood coagulation is still controversial. The aim of this study was to investigate the effect of intravenous propofol infusion on platelet aggregation and blood coagulation.Methods Forty-one ASA I or II patients of both sexes (22 male, 19 female) aged 24-56 yr weighing 46-75 kg scheduled for elective upper abdominal surgery were enrolled in this study. The patients were premedicated with intramuscular atropine 0.5 mg and phenobarbital 0.1 g. Anesthesia was induced with fentanyl 4 ?g?kg-1 , and propofol 2 mg?kg-1 . Tracheal intubation was facilitated with vecuronium 0.15 mg? kg-1 . The patients were mechanically ventilated (VT 8-12 ml?kg-1 , RR 12 bpm). Anesthesia was maintained with TCI of propofol and intermittent IV boluses of fentanyl and vecuronium. Propofol was infused with a TCI system ' Diprifusor' . The target effect site concentration was set at 4 ?g ? ml-1 . Platelet aggregation and intracellular calcium ion concentration [Ca2+ ]i were measured and thromb-elastography (TEG) and coagulation function tests (APTT, TT, PT, FIB) were performed before induction (T0, baseline), at 30, 60, and 120 rain after induction (T1 , T2 , T3) . Results (1) The maximum platelet aggregation rate was significantly decreased at T1,3 as compared to the baseline (T0) ( P