Method for assisting piloting of a rotorcraft at high altitudes by virtue of delivering mechanical power originating from an electric power plant of said rotorcraft, said rotorcraft comprising: - at least one rotor (3, 4); - a hybrid power plant provided with said electric power plant and a thermal power plant that are connected to said at least one rotor (3, 4) by a mechanical transmission chain, said thermal power plant comprising at least one heat engine (11, 12), at least one engine computer (13, 14) of said at least one heat engine (11, 12) and at least one tank containing fuel supplying said at least one heat engine (11, 12), said electric power plant comprising at least one electric machine (21, 22) as well as at least one control device (23, 24) for controlling said at least one electric machine (21, 22) and at least one electric energy storage device supplying said at least one electric machine (21, 22) with electric energy; - a display device; and - a computer, characterised in that said method comprises the following steps: - defining a take-off point (PtD) of said rotorcraft and its initial altitude; - defining a target point (PtO) for said rotorcraft and its final altitude; - determining, by means of said computer, a first maximum altitude that can be reached by said rotorcraft using only said thermal power plant, according to a first altitude law, based on an initial total mass of said rotorcraft before take-off, on the initial amount of fuel on board that is contained in said at least one tank, on an atmospheric pressure and on an air temperature, and on an ageing state of said at least one heat engine (11, 12); - estimating, by means of said computer, a second maximum altitude that can be reached by said rotorcraft using said thermal power plant and said electric power plant, according to a second altitude law, based on said first maximum altitude, on said initial total mass of said rotorcraft, and on a predicted amount of intermediate electric energy available in said at least one electric energy storage device at said first maximum altitude; - displaying information relating to a comparison between said final altitude and said second maximum altitude on said display device. - at least one rotor (3, 4); - a hybrid power plant provided with said electric power plant and a thermal power plant that are connected to said at least one rotor (3, 4) by a mechanical transmission chain, said thermal power plant comprising at least one heat engine (11, 12), at least one engine computer (13, 14) of said at least one heat engine (11, 12) and at least one tank containing fuel supplying said at least one heat engine (11, 12), said electric power plant comprising at least one electric machine (21, 22) as well as at least one control device (23, 24) for controlling said at least one electric machine (21, 22) and at least one electric energy storage device supplying said at least one electric machine (21, 22) with electric energy; - a display device; and - a computer, - defining a take-off point (PtD) of said rotorcraft and its initial altitude; - defining a target point (PtO) for said rotorcraft and its final altitude; - determining, by means of said computer, a first maximum altitude that can be reached by said rotorcraft using only said thermal power plant, according to a first altitude law, based on an initial total mass of said rotorcraft before take-off, on the initial amount of fuel on board that is contained in said at least one tank, on an atmospheric pressure and on an air temperature, and on an ageing state of said at least one heat engine (11, 12); - estimating, by means of said computer, a second maximum altitude that can be reached by said rotorcraft using said thermal power plant and said electric power plant, according to a second altitude law, based on said first maximum altitude, on said initial total mass of said rotorcraft, and on a predicted amount of intermediate electric energy available in said at least one electric energy storage device at said first maximum altitude; - displaying information relating to a comparison between said final altitude and said second maximum altitude on said display device.