This study investigates changes in intraocular pressure (IOP) in rabbits during head-down tilt (HDT), which is commonly used as an experimental model to simulate microgravity. IOP was measured by the needle insertion technique (IOPNEEDLE) and Tono-pen tonometry (IOPTONO-PEN). Although the absolute value of the IOPTONO-PEN was significantly smaller than that of the IOPNEEDLE, a significant correlation (r = 0.99) was observed between them. A linear regression analysis yielded an equation as follows: IOPTONO-PEN = 0. 67 IOPNEEDLE - 0.67. Both the IOPNEEDLE and the IOPTONO-PEN changed depending on the tilt angle. Tilting from horizontal (0 degrees) to 75 degrees head-down increased the IOPNEEDLE and the IOPTONO-PEN by 7.3 +/- 0.8 (mean +/- SEM) mmHg and 4.4 +/- 1.3 mmHg. The IOPNEEDLE elevated from 13.1 +/- 1.3 to 16.9 +/- 1.0 mmHg immediately after the onset of 45 degrees HDT and then gradually declined. The value of the IOPNEEDLE during 8 h of HDT was significantly higher than the value in the control animals, which were kept at the horizontal prone position throughout the experiment. Similar findings were observed in the IOPTONO-PEN. These results suggest that the needle insertion technique and the Tono-pen tonometry are both useful for measuring IOP in rabbits.