Purpose: The present study was undertaken to determine the 24-hour pattern of intraocular pressure (IOP), and to reveal the effect of unilateral decentralization on circardian rhythm of IOP in eyes of rats. Methods: Nine male Sprague-Dawley rats were exposed to a 12-hour light/dark cycle for 3 weeks. 24-hours IOP pattern was measured at six time points (8,12,16,20,24, and 4 circardian time) by Tonopen XL tonometer in awake state. After unilateral sympathectomy (cervical ganglionectomy), IOP measurements were performed at the same time points on post-operation 1, 2, 3, and 4 weeks. Results: 24-hour IOP pattern of rats showed a rhythmic pattern that appeared to be sinusoidal with 12-hour light/dark cycle. IOPs were low during the light phase and high during dark phase. After unilateral sympathectomy, the circardian rhythm of IOP was maintained in control eyes. But in sympathecotmized eyes, the circardian thythm of IOP was loss. Conclusions: These results show that sympathetic nervous systems contribute to the circardian rhythm of IOP in rats.