The tip diameter of the standard Goldmann applanating prism was reduced from 7.0 mm to 4.0 or 4.5 mm, but the endpoint of intraocular pressure measurement-applanation of a circle of cornea having a diameter of 3.06 mm-was not changed. The "minified" tonometers exhibited standard mechanical calibration characteristics in three standard configurations. The 4.5-mm tonometer was calibrated for cynomolgus monkey eyes in vivo by open and closed stopcock manometry. By open stopcock manometry, between pressures of 4 and 70 mm Hg, IOP(tonometer) = 1.01 IOP(manometer)-0.72, with very little scatter or curvature. During closed stopcock manometry, between pressures of 5 and 55 mm Hg, IOP(tonometer)=1.07 IOP(manometer)-1.32, again with very little scatter or curvature. The tonometer elevated manometric IOP by an average of about 7.5%; the percentage pressure elevation decreased slightly as pressure increased. The 4.0-mm tonometer was calibrated against a standard tonometer in living human eyes. Over the pressure range from 0 to 75 mm Hg, IOP(minitonometer)=0.98 IOP(standard tonometer)+ 0.82, with very little scatter or curvature. Similar comparison of two standard tonometers showed a virtually identical relationship and scatter. We conclude that minified Goldmann tonometers provide accurate and reprodible measurements of IOP in cynomolgus monkey and human eyes.