The reduction of bleeding during surgery can be aimed at for two reasons: to facilitate the operation by providing a bloodless field and/or to avoid blood loss requiring compensation by transfusion. These aims can be reached by reducing blood pressure or by acting on blood-clotting. For inducing hypotension sodium nitroprussiate and isoflurane are mainly used. Complex and precise monitoring is required for patient safety. Two drugs have been used to diminish bleeding: desmopressin and aprotinin. With the latter an important reduction (40 to 50%) of the blood loss has been achieved.