Abstract: A spacer for delivering a maximum amount of a therapeutic agent to the lungs. The spacer may be connected in the inspiratory line running from a mechanical ventilator or hand held and operated manually by the nonintubated, spontaneously breathing patient or the intubated, spontaneously breathing patient. The spacer includes the following features to maximize dispersion of the agent into the fluid flow: a structure for spraying the therapeutic agent in a retrograde fashion to prevent loss of the therapeutic agent to the expiratory limb of the respiratory circuit; a frustoconical shape that reflects the cone-like pattern of an aerosol spray to minimize impaction of the sprayed agent on the inner surfaces of the spacer; and a baffle to readily mix the fluid with the therapeutic agent and deflect fluid flow along the inner walls of the spacer where the therapeutic agent may otherwise collect.