After injection of urokinase (UK) in normal rabbits and in rabbits with ligated liver, spleen and kidneys, the inactivation rate constant of UK was evaluated by using the disappearance rate of fibrinolytic activity from plasma. A peripheral intravenous injection of UK in normal rabbits induced significant but transient fibrinolytic activity, whereas intraportal injection of UK induced no fibrinolytic activity. However, in rabbits with ligated liver, spleen and kidneys, the fibrinolytic activity sustained for a long period, i.e. the inactivation rate constant of UK was 6 times smaller compared with normal rabbits. The implications of these results are discussed.