Abstract: The hydraulic fluid contained in a control circuit such as an aircraft circuit is replaced by expelling the spent fluid with new fluid. For this purpose the tank (10) is disconnected from the circuit after depressurizing and emptying it. The suction branch (18) is then connected to a new fluid supply (42) to return branch (34) a casing of a high pressure filter (30) and a reservoir (48) for recovering the spent fluid. The fluid is transferred sectionwise by operating pumps (22, 4) and by manipulating the control devices (28a, 28b). A check on the transferred fluid volume makes it possible to ensure that the replacement has taken place. Following a disconnection of the supply device (42) from the reservoir (48), the tank (10) is connected to the control circuit, filled and repressurized.