Abstract: The device comprises a closed container with a breathing tube and a means for variably adjusting the size of an aperture into the container. A transparent inclined tube is mounted on the container and houses a moveable air flow indicating member. The tube has a passage at the top leading into the container and a vent at the bottom of the tube allows air to enter to move the flow indicating member up the inclined tube. Air drawn from the container by inspiring through the breathing tube is replaced in part by air entering through the variable aperture means and by the air entering through the vent. By adjusting the size of the variable aperture, one can determine inspiratory capacity as correlated with the amount of air necessary to move the flow indicating member to the top of the tube or to an intermediate position in the tube.