Abstract: A nasal cannulae is described for insufflating a treating gas into one nostril and measuring at least one gaseous component of exhaled breath in the other nostril of a living body. The cannulae includes a wall member cooperating with a hollow body of the cannulae to define inhalation and exhalation manifolds and sealingly engaging the hollow body to provide a gas-tight seal for positively preventing fluid communication between these manifolds. A hollow nasal prong communicates with each manifold and is positioned and shaped to be received in a corresponding nostril. The wall member may be integrally molded with the body of the cannulae when it is made or a conventional cannulae may be modified by insertion of a wall member to create separate manifolds. The nasal prongs may have the same or different lengths.