Changes in fibrinolysis, platelets and coagulation during hypothermia were investigated in anesthetized dogs. Strong activation of plasma fibrinolysis was observed in the hypothermia group. The maximum fibrinolytic activity determined with standard fibrin plates was markedly higher than that of the control sample (mean values: 67 and 15 mm2/0.03 ml plasma, respectively). On the other hand, no fibrinolysis was observed by the plasminogen free plate method. Activation was not observed in the control group or the hypothermia group premedicated with chlorpromazine. The platelet count was decreased from 100% to 14.7% and the collagen induced platelet aggregability from 61% to 2.3% during hypothermia. Recovery from both reductions occurred on rewarming. In the control and premedicated groups, the recalcification time was reduced progressively (from 93.6 to 60.9 sec and from 101.1 to 71.8 sec as mean times, respectively). The tendency towards shortening of the recalcification time was not related to the core temperature.