A method and apparatus for priming an extracorporeal blood circuit, in which the patient end of an arterial line (79) is connected to a first discharge port (61), and the patient end of a venous line (87) is connected to a second discharge port (62). The two discharge ports are connected to a used dialysate line which connects a dialyser (33) to a drain. The arterial and venous lines are filled with a priming fluid, while the air contained in the arterial and venous lines is evacuated partly through the first discharge port and partly through the second discharge port. Two check valves (65, 66) prevent flow from the used dialysate line towards the two discharge ports. The invention reduces the risk of errors on the part of an operator readying the priming configuration, as well as the risk of contamination of the extracorporeal circuit during the priming phase.
