A plasma increase of liver enzymes has been recently reported in patients receiving heparin therapy. In this study we have evaluated the toxic effect of heparin infusion in the whole rat and in the isolated and perfused rat liver. No variation of plasma enzymes was observed in heparin-treated rats (10 IU per gram body weight, daily for 12 days). The heparin addition in the perfusion medium (5,000 IU in all) has shown no difference in the kinetics of hepatic enzymes release and in the other parameters of liver function. These data do not confirm a liver-toxic effect of heparin in the rat.