The patency-rate of venous bypasses depends - beside other important factors - upon the integrity of the transplanted vessel, especially on the condition of the tunica intima. Under this aspect we examined by light microscopy a total of over 1100 slices of human as well as animal vein-segments which had been stored in arterial heparinized blood or Euro-Collins-Solution at 4 degrees C, 20 degrees C and 37 degrees C up to 90 minutes. The results show that veins should be used immediately after harvesting. If intraoperative storage is necessary blood at a temperature between 20 degrees C and 37 degrees C is the better medium for storage periods up to 90 minutes.