The possible mechanisms for shortening prothrombin time (PT) and activated partial thromboplastin time (APTT) were investigated using citrated plasma from rats and dogs in vitro, especially focusing on increased fibrinogen concentrations. When purified canine fibrinogen was added to citrated canine plasma at final concentrations of 2, 4 and 8 mg/mL, PT and APTT were significantly shortened. The increased concentrations of clottable fibrinogen in the test system were confirmed by markedly shortened thrombin time (TT). In citrated rat plasma, while purified rat fibrinogen had no effect on PT or APTT at final concentrations of 2, 4 and 8 mg/mL, it did shorten TT. These results suggest that an increased concentration of fibrinogen is a possible mechanism to shorten PT and APTT in dogs, but not in rats.