Purpose.Protracted daily lighting cycles do not promote abnormal ocular enlargement in infant monkeys as they do in a variety of avian species. However, observations in humans suggest that ambient lighting at night may reduce the efficiency of the emmetropization process in primates. To test this idea, we investigated the ability of infant monkeys reared with continuous light to compensate for optically imposed changes in refractive error.Methods.Beginning at about 3 weeks of age, a hyperopic or myopic anisometropia was imposed on 12 infant rhesus monkeys by securing either a −3 D or +3 D lenses in front of one eye and a zero-powered lens in front of the fellow eye. Six of these monkeys were reared with the normal vivarium lights on continuously, whereas the other six lens-reared monkeys were maintained on a 12-h-light/12-h-dark lighting cycle. The ocular effects of the lens-rearing procedures were assessed periodically during the treatment period by cycloplegic retinoscopy, keratometry, and A-scan ultrasonography.Results.Five of six animals in each of the lighting groups demonstrated clear evidence for compensating anisometropic growth. In both lighting groups, eyes that experienced optically imposed hyperopic defocus (−3 D lenses) exhibited faster axial growth rates and became more myopic than their fellow eyes. In contrast, eyes treated with +3 D lenses showed relatively slower axial growth rates and developed more hyperopic refractive errors. The average amount of compensating anisometropia (continuous light, 1.6 ± 0.5 D vs. control, 2.3 ± 0.5 D), the structural basis for the refractive errors, and the ability to recover from the induced refractive errors were also not altered by continuous light exposure.Conclusion.Ambient lighting at night does not appear to overtly compromise the functional integrity of the vision-dependent mechanisms that regulate emmetropization in higher primates.