In a Cross-Over study the influence of a hyper- and isovolaemic haemodilution on blood fluidity, blood flow in the macro- and microcirculation as well as on tissue oxygen tension (M. tibialis anterior) was investigated in n=8 patients with arterial occlusive disease stage II according to Fontaine. In both cases 500 ml hydroxyethyl starch (HES 200/0.5 10%, Fresenius AG) were infused within a period of 60 minutes. In case of isovolaemic haemodilution 250 ml hydroxyethyl starch were given over 20 minutes, then 250 ml blood withdrawn (over 10 min), 250 ml hydroxyethyl starch given and at least again 250 ml blood withdrawn.Prior to and 1, 3, 6 and 24 hours after haemodilution the rheological parameters (haematocrit, plasma viscosity, erythrocyte aggregation, erythrocyte rigidity and spontaneous thrombocyte aggregation) as well as the oxygen partial pressure histogram in the tibialis anterior muscle were measured.Blood pressure and heart rate did not change during and after haemodilution. Painfree walking distance increased after isovolaemic (105% after 1 h) more than after hypervolaemic (31% after 1 h) haemodilution. The fluidity of blood shows a significant improvement only after isovolaemic haemodilution; plasma viscosity decreased about 5%, erythrocyte aggregation about 22% and haematocrit about 16%. 24 h after haemodilution the rheological parameters reached initial values.The intramuscular oxygen partial pressure increased after hypervolaemic haemodilution about 35%, after isovolaemic haemodilution about 45%.