The purpose of this study was to examine whether there is a circadian rhythm of intraocular pressure (IOP) in the Lewis rat. Therefore, 20 Lewis rats (40 eyes) were subjected to an alternating 12-hour-light and 12-hour-dark cycle. Using a Tono-Pen 1, IOP was measured every 2 h between 6 a.m. and 12 midnight and every 3 h between 12 midnight and 6 a.m. for 2 consecutive days. These measurements in the Lewis rat revealed a reproducible circadian rhythm with a peak at 8 p.m. and a trough at 6 a.m. In conclusion, when using the Lewis rat as an animal model in longitudinal studies that involve the IOP, the IOP should be measured at the same time each day to correct for diurnal variations. Additionally, Lewis rats may be a useful model to study the causes of circadian rhythms of IOP and for pharmacologic studies of new glaucoma medications.