Acute hemodilution has been performed in 46 patients immediately before major gastrointestinal surgery. Clinical, hemodynamic, respiratory and metabolic parameters have been studied and compared with results in 66 patients, submitted to similar operations under equal conditions without hemodilution. Two kinds of hemodilution have been used: normovolamic hemodilution by bloodexchange with 5% human albumin and moderate hypervolamic hemodilution by bloodexchange with equal parts of Dextran 60 and PPL.1. The first mentioned kind of hemodilution and method of blood-exchange was superior in regard to stability of cardiovascular behavior. 2. The general tolerance of anesthesia and surgical trauma was superior in the nonhemodiluted group, but was good or tolerable also in the hemodiluted group in 82,6%. Side effects, more frequently and more pronounced in the hemodiluted group concerned to hypo- and hypertensive reactions, vasoconstriction, hypokaliemie and cardial arrhythmics. Exclusively in the hemodiluted group ST-lowering in the EKG has been stated in some patients. These reactions have been estimated as a sign of marginal compensatory capacity in the individual cardial situation and were reversibel by blood transfusion. These observations strengthen the necessity for accurate monitoring in hemodiluted patients. Moreover it is recommended to restrict hemodilution in the level of Hk 27-28%. Under these conditions acute normovolamic hemodilution is recommended further for a wider clinical use.