Abstract: The invention relates to a system for supplying noxious gas into an exposure chamber containing plants which are to be exposed, under controlled conditions over long periods of time, to an atmospheric gas which is given a defined composition by the introduction of the noxious gas or gases comprising:a circuit arrangement for controlling the mixing of the atmospheric gas with the noxious gas(es) including a respective mass flow regulator equipped with a subsequently connected shut-off valve provided in the respective gas inlet conduits with the shut-off valves being switched jointly; a premixer stage connected to the gas inlet conduits for homogeneously mixing the atmospheric and noxious gases; a buffer reservoir for the thus composed mixture of atmospheric and noxious gases; and a computer controlled dosaging device for the mixture of atmospheric and noxious gases to be introduced into the exposure chamber.