Comparisons were made of measurements obtained in horses, using 2 applanation tonometers in vivo and in vitro. In vitro comparisons indicated that although neither instrument accurately recorded intraocular pressure (IOP), compared with manometric measurements, results of both instruments indicated linear digression from manometric IOP values that could readily be corrected, thereby accurately estimating IOP in horses. For tonometer 1 (MacKay-Marg), calculated actual IOP = 1.48 - 0.9 mm of Hg; and for tonometer 2 (Tono-Pen), calculated actual IOP = 1.38 + 2.3 mm of Hg. The coefficients of determination (r2) values were markedly high (0.99 for both equations). In vivo comparisons in clinically normal horses did not reveal significant differences in measured IOP between the 2 instruments, and IOP was not altered from baseline after auriculopalpebral nerve block. Mean (+/- SD) IOP in clinically normal horses was 23.5 +/- 6.10 mm of Hg and 23.3 +/- 6.89 mm of Hg, for tonometers 1 and 2, respectively.