The injection of certain antigens into the anterior chamber (AC) of the eye results in the induction of antigen-specific suppressor T cells (Ts cells), which inhibit systemic delayed-type hypersensitivity (DTH). We have previously shown that down-regulation by Ts cells after AC injection with 2,4,6-trinitrophenol (TNP)-coupled spleen cells (TNP-Spl) is initiated by the intraocular activation of Ts inducer cells. These cells activate T suppressor-effector cells in the spleen that are responsible for suppressed DTH. With dark- and light-reared mice (Balb/c), we show that visible light has a direct effect on the intraocular T cell reaction that leads to systemic suppression. Our results show that if light is prevented from reaching the eye by dark rearing, by placing light-reared animals in the dark after AC injection, or by closing the eyelids of light-reared animals after AC injection, Ts cells are not activated. We show that light is responsible for establishing conditions in the eye that cause the preferential activation of Ts cells. The intraocular conditions established by light are not developmentally mandated as is visual development, but can be eliminated in adult light-reared animals by placing them in the dark for 18 h after AC injection. These conditions can also be induced in adult dark-reared animals by returning them to the light for just over 24 h before AC injection. These studies have important implications for understanding intraocular immune responses and possibly for the treatment of eye disease.