BACKGROUND: Activated coagulation time (ACT) is commonly used to guide heparin and protamine dosing during cardiovascular surgery. There are many factors that influence the ACT such as time of test, hemodilution, temperature, aprotinin and etc. We considered the other factor that influence the ACT, the route of blood sample. METHODS: This study included 40 patients who were scheduled for cardiac surgery. Whole blood was sampled through arterial and central venous line at 10 minutes after surgical incision and heparin administration. The ACT was measured with Hemochron 801 blood coagulation timer with 12 mg of celite surface activator. RESULTS: At 10 minutes after surgical incision and heparin administration, arterial blood and venous blood ACTs were 127 20, 537 214 seconds and 118 18, 496 145 seconds respectively (p<0.05). CONCLUSIONS: We conclude that the venous blood ACT is more less than arterial blood ACT during cardiovascular surgery.