Comparative intraocular pressure measurements made with the pneumatonograph and the Goldmann tonometer showed close agreement with a statistically higher mean pressure when using the pneumatonograph. Significant changes in mean pressure differences between the two instruments were seen upon changing the pneumatonograph calibration and with change in the order of testing (which instrument was used initially). However, transient biologic changes in an individual eye producing minute-to-minute fluctuations in intraocular pressure may be equal to or larger than differences in measurement between instruments.