In a gas-supply system for internal-combustion engines, a pressure-reducing valve is arranged between the reservoir of the gas and a distribution manifold or rail that feeds the gas to a series of electromagnetically controlled injectors associated to the various cylinders of the engine. The pressure-reducing valve has an internal chamber for collection of any leakage of gas which is connected to the intake manifold of the internal-combustion engine so as to enable the leaked gas to be discharged in the engine. A solenoid shutoff valve is arranged in the connection to the intake manifold and is opened automatically upon ignition of the engine.