Nitric oxide (NO) has been proposed to mediate light-adaptation in the vertebrate retina. However, the in vivo NO concentration in the retina is not known. We measured NO in the vitreous adjacent to the retina of the rat eye using NO-selective electrodes under various light conditions. The rats were kept under a 12:12 h light/dark cycle with lights on from 08:00 to 20:00 h. NO during the daytime in the light remained constant at 0.85+/-0.41 microM (n=10), and decreased after dark-adaptation, while NO during the nighttime in darkness was 0.55+/-0.27 microM (n=5), and increased in the light. The vitreous NO initially increased rapidly to flicker, but then decreased as the flicker continued. We found that the diurnal change of NO in the vitreous depended on the ambient light condition.