Abstract: A method for administering a liquid medical substance in a vein. The method comprises inserting a plurality of tubular ducts into a target venous lumen, conducting a liquid medical substance via at least one of the plurality of tubular ducts (106, 215, 419, 602) for the administering thereof toward a venous wall (110, 410) at the target venous lumen in an ejection pressure, quantity and direction, and withdrawing a residue of the liquid medical substance from the target venous lumen, in a suction pressure, quantity and direction, for the conducting thereof via at least one different of the plurality of tubular ducts (130, 240, 400). A relation between the ejection pressure, quantity and direction and the suction pressure, quantity and direction limits the extent of the administering within the venous lumen to a predefined space.