The problems encountered in measuring intraocular pressure in eyes containing a compressible gas and the problem of determining the effect of changes in ambient atmospheric pressure on such eyes have been studied experimentally in enucleated human eyes and living rabbit eyes, and studied theoretically using the Friedenwald pressure-volume relationship, Boyle law, and the Goldmann equation of aqueous humor inflow and outflow. It has been concluded that high displacement tonometry, such as Schiötz tonometry will give false low measurements of intraocular pressure under these circumstances, and that there is no reliable way to correct these erroneous measurements other than to carry out tonometry with a low displacement instrument such as the Goldmann tonometer, the Perkins tonometer, or the Mackay-Marg tonometer. It was noted that large changes in intraocular pressure occur during rapid changes in ambient atmospheric pressure. It was concluded from theoretical considerations that persons with an eye containing 1 cu cm or more of a compressible gas should be advised to avoid air travel.