The middle ear (ME) pressure balance is an important contributor to the maintenance of ME health. ME pressure is a measure of the number of contained gas moles and processes that add or remove gas moles from the relatively fixed volume ME cavity change its pressure. Three routes for ME gas exchange have been identified; 1) passive, gradient-dependent species exchange with the local blood via diffusion across the ME mucosa; 2) passive, gradient-dependent species exchange with the ambient environment via diffusion across the tympanic membrane, and 3) bolus exchange of mixed gases between the nasopharynx and ME during transient openings of the Eustachian tube. Total ME pressure and the trajectory of ME pressure under specified conditions depend on the relative rates of species gas exchanges across these pathways. The ME airspace can be subdivided into two compartments, the anterior tympanum and the posterior mastoid air cell system (MACS). Theory suggests that the transmucosal gas exchange rate for the ME (transMEM) may be different for the tympanum and the MACS and this possibility has important implications with respect to the role of the MACS in ME pressure regulation and protection from ME disease. This study measures the rate of inert gas exchange across the mucosa of the two ME compartments in adults. These data will be fitted to a model of gas exchange parameterized based on the relative surface areas and volumes of the two compartments, and the hypothesis that volume gas exchange across the MACS mucosa is less than that for the tympanum will be tested. The results will be used to reject or confirm one mechanism by which large MACS volume protects from otitis media under certain conditions.