Monocular closure surgically performed during development by suturing the eyelids induced less hyperopia in the closed eye of light reared kittens (+0.95 Diopters) in comparison to the closed eyes of lid sutured dark reared kittens (+2.61 D). The normal control cats were also slightly hypermetropic (+0.69 D). While a certain proportion of myopic eyes was found in the monocularly closed light reared kittens and in the normal control cats, no one eye was myopic either in the operated or nonoperated dark reared kittens. Lid suture was found to considerably increase the hyperopia also in adult cats. The incidence of astigmatism was 47.0% for the closed eye in the lid sutured light reared kittens and 64.7% for their open eyes; for the lid sutured dark reared kittens the proportions were 45.4% and 54.5% respectively. The incidence of astigmatism was 20.2% for the eyes of the normal control cats. Axial length of the closed eye of light (19.00 mm) and of dark (19.45 mm) reared kittens was smaller in comparison to that of the normal control cats (20.38 mm). The corneal curvature of the closed eyes of light (8.09 mm) and of the dark (8.64 mm) reared kittens was flatter than in the normal controls (7.11 mm). It is concluded that lid closure in kittens has a corneal effect, accentuating the tendency for hypermetropia naturally occuring in the dark. In the light lid closure results in an imbalanced combination of the corneal and the (axial?) visual deprivation effects, causing a considerable variability in the refractive error with a tendency for cancellation of the naturally occuring hyperopia.