Circadian rhythms in aqueous protein concentration (APC) and intraocular pressure (IOP) were examined in Dutch rabbits using the laser flare-cell meter and a pneumatonometer. The reproducibility and seasonal changes in circadian rhythm were also investigated. Rabbits were trained to a lighting cycle of 12-hour light and 12-hour darkness. The mean value of reproducibility coefficients obtained with re-examination at 2-week intervals was 21.9% in the reading of the laser flare-cell meter. APC was reduced during the dark phase, and increased rapidly and remained high during the light phase. On the other hand, IOP showed an inverse circadian rhythm which was negatively correlated with that of APC. The total average of APC throughout 24 hours was 39.1 +/- 28.3 (mean +/- SD) mg/100 ml equivalent of bovine serum albumin, which agreed with the previous data. Seasonal changes in circadian rhythm were not statistically significant either in APC or in IOP.