Anticoagulants are effective for preventing both venous and arterial thrombosis. Although antithrombotic agents have been reported to reduce thrombin formation, to our knowledge, the relation between blood viscosity (BV) and antithrombotic agents has not been examined. We examined the effects on whole BV of various dosages of the antithrombotic agents-heparin sodium and argatroban. Thirty microliters of either drug was added to 3-ml samples of blood obtained from healthy male volunteers. Whole BV was then immediately examined with an oscillation-type viscometer. When either agent was added, BV decreased and coagulation time increased dose dependently. BV was 4.5+/-0.3 mPa.s in untreated blood but decreased in a dose-dependent manner to a minimum of 2.5+/-0.3 mPa.s with heparin sodium and decreased dose dependently in a sigmoid manner with argatroban. Because thrombin generation is inhibited in all antithrombotic therapies, this inhibition might be reflected by changes in BV. Our results suggest that BV in accordance with blood coagulability are indexes of thrombotic tendency and that decreasing BV prevents life-threatening thromboembolic conditions.