Ocular tension recorded by pneumatonography was estimated at 4-hour intervals during six consecutive 24-hour periods in 8 New Zealand rabbits. The animals had been acclimatized for more than 8 weeks to artificial light from 08.00 to 20.00 h and dark for the other 12 h. Inter- and intrarabbit consistency was enough to allow the generalization that the lowest pressure occurred at noon (mean -0.8 mm Hg from baseline average), while the highest pressures (mean + 1.2 mm Hg above baseline average) occurred at 16.00 and 20.00 h. Only three other studies have measured rabbit ocular tensions throughout 24 h, but for shorter periods: one observed a light-induced rise as in the present study, but the other two studies found a rise during darkness.