Abstract: A variable orifice pulse valve has a small internal volume for providing small, accurately measured pulse volumes of gas. The valve may be employed to supply a very small quantity of a therapeutic gas or a diagnostic gas into the breathing gases of a patient. The valve has a housing with an inlet for receiving the gas and an outlet providing the pulsatile discharge of the gas. A chamber, formed in the housing, has a first opening in fluid communication with the inlet and a second opening in fluid communication with the outlet. The intermediate chamber is formed to have a small volume compared to the minimum pulsatile gas discharge volume of the valve. A spring biases a variable orifice member toward the first opening and a solenoid moves the variable orifice member away from the first opening to form an orifice that meters a dose of gas into the valve. A sealing member is spring biased into sealing engagement with the second opening.