In light-dark entrained rabbits, the intraocular pressure (IOP) increases around the onset of dark due to the increased activities of ocular sympathetic nerves. This circadian elevation of IOP can be eliminated by exposing the rabbits to constant white light in the early subjective dark phase. The spectral effectiveness of light causing the reduction in IOP was studied using lights selectively filtered through various optic filters. Light which passed through a long-pass filter, with a cut-on wavelength at 570 nm or 530 nm, caused no change of the circadian IOP elevation. A significant reduction of the IOP elevation was observed with filtered light via the 495 nm long-pass filter and, to a larger extent, with light via the 475 nm long-pass filter. Short-wavelength light (380-480 nm, peak at 432 nm) passed through a band-pass filter completely eliminated the circadian IOP elevation. The aqueous humor concentration of norepinephrine (NE) under the short-wavelength light was significantly lower than the NE concentration under the filtered light via the 530 nm long-pass filter. Sensation of the short-wavelength light plays an important role in synchronizing the circadian elevation of IOP in rabbits.