We have proposed that the susceptibility of some eyes to glaucomatous damage is due to an abnormal weakness of the structural support of the optic nervehead. To verify this hypothesis and, if it proves correct, to provide an early test of glaucoma, we have developed a noninvasive method of measuring the optic nervehead compliance. We present here the first in-vivo measurements that were performed in a normal beagle dog and dogs with congenitally elevated intraocular pressure (IOP) over different periods of time. The quality of the measurements was tested and the reproducibility of the optic nervehead displacement was +/- 15 microns. The optic nervehead displacement was found to be higher in the older dogs that also sustained ocular hypertension for a longer period. These results were compared with ophthalmoscopic observations of the optic disc. The present study indicates that the method is applicable to the measurement of the optic nervehead compliance and that the structural support of the optic nervehead may vary.