A spacer for delivering a medicinal substance is provided having a chamber having a tubular body defining a first reservoir, the top end of the body narrowing to define a single unobstructed central chamber aperture, a base for the body having an inlet for admission of the substance into the first reservoir, a vented member connected atop the body, a mouthpiece connected atop the vented member and partly defining a second reservoir, the mouthpiece having an outlet for withdrawal of the substance from the second reservoir The spacer also has a valve within the chamber having a self-supporting central domed portion and defining a central cross slit at its peak, the domed portion opening during inspiration and closing during expiration, and a flat ringed portion surrounding the domed portion with a circumferential free edge, the diameter of the ringed portion being approximately twice the diameter of the domed portion.
