The effect of constant light and constant darkness on intraocular pressure (IOP) in goats has not been investigated. We hypothesized that IOP variations would differ between goats kept under a cycle of 12 hours of light and 12 hours of darkness (LD), constant darkness (DD), and constant light (LL). To test this hypothesis, goats were exposed to these conditions for five days (LD, 30 goats; DD, 10 goats; LL, 10 goats). IOP was measured by applanation tonometry at 9 a.m. (beginning of photophase in LD) and 9 p.m. (beginning of scotophase in LD) on the fourth and fifth days of exposure. We found that changes in mean IOP from 9 a.m. to 9 p.m. differed significantly between groups (χ2(2) = 23.04, p < .0001). Most goats in LD showed a regular pattern of higher IOP in the morning and lower IOP in the evening, whereas those in DD and LL did not follow this pattern. In LD conditions, mean IOP was 2.4 mm Hg lower at 9 p.m. than at 9 a.m. (95% confidence interval for the difference (CI): -2.8 to -1.9 mm Hg, p < .0001). In DD conditions, mean IOP did not differ between 9 p.m. and 9 a.m. (CI: -0.9 to 0.8 mm Hg, p = .90). In LL conditions, it was 0.6 mm Hg lower at 9 p.m. (CI: -1.5 to 0.2 mm Hg, p = .12). Our results indicate that IOP in goats kept in LD is higher in the morning than in the evening, and that IOP variations are reduced in goats kept in DD and LL. These results suggest that exposure to alternating periods of light and darkness is important for maintaining rhythmic variations in IOP in this species.