Tree shrews were monocularly deprived (MD) from the day of eye opening for periods of 15, 30, 45, 60 and 75 days. The initial structural change after 15 days of MD was a flattening of the corneal curvature in the deprived eye causing a hyperoplc increase in refraction, relative to the fellow control eye. A relative myopia was first observed after 30 days of deprivation and increased as the length of MD increased. Animals monocularly deprived for 75 days consistently showed high degrees of myopia (>-10D). An increase in vitreous chamber depth was found after 30 days of deprivation and continued to increase, relative to the control eye, throughout the developmental period under investigation. There was a strong correlation (r = 0.84) between increase in vitreous chamber depth and the amount of experimentally-induced myopia. Anterior chamber depth was shallower in the deprived eyes of all animals. The crystalline lens was also consistently thinner in the deprived eye. Based on optical modeling, the observed myopia was consistent with the changes in ocular component dimensions. The susceptible period for experimental myopia begins about 15 days after eye opening. © 1992.