The diurnal variations in intraocular pressure in the rabbit were measured during a period of 12 months in a cross-sectional study. The average intraocular pressure of all the measurements taken during the year was 19.6 +/- 2.0 mmHg (mean +/- S.D.; n = 1957). The daily curve showed a continuous rise during the day from 18.2 +/- 1.7 mmHg (n = 175) at 0800 hr to 20.6 +/- 1.8 mmHg (n = 167) at 1700 hr. During the month of June, when the measurements were extended until midnight, this rise in intraocular pressure continued until 2200 hr, reached a level of 22.5 +/- 1.6 (n = 16), and started to decline. A similar daily pattern was observed throughout the year. The annual curve showed two peaks, during the summer and winter seasons, with a daily average of 20.2 +/- 2.1 (n = 404) and 20.2 +/- 2.0 (n = 638) respectively. The lowest average daily IOP value was obtained during the spring and was 18.8 +/- 2.0 (n = 509). An intermediate level between the summer/winter and the spring values was obtained during the autumn and was 19.4 +/- 1.7 (n = 406).