To examine whether collagen crosslinking is important for the regulation of refractive development, tree shrews were treated with agents that block collagen crosslinking [β-aminoproprionitrile (β-APN), or D-penicillamine (DPA)] and underwent monocular deprivation (MD) of form vision by eyelid closure to induce myopia. MD began on the first day of visual exposure and continued for 75 days. After 15-20 days of visual exposure, daily intraperitoneal injections of β-APN (β-APN MD animals, (n = 5) or DPA (DPA MD animals, (n = 5) were administered for 17-21 days. β-APN open animals ((n = 5) received the same injection schedule, but both eyes remained open. Saline MD animals ((n = 5) received i.p, saline and MD. At 75 days of visual exposure, the MD eyes of β-APN treated tree shrews were highly myopic (-23.6±3.03 D) in comparison to their open control eyes. This was markedly greater than the difference in saline MD animals (-11.0±0.8 D). DPA MD animals showed a relative myopia of -14.3±2.2 D. The structural correlate of the myopia, elongation of the vitreous chamber in the deprived eyes relative to the control eyes, was significantly greater in the β-APN MD animals (0.85±0.09 mm) than in the saline MD group (0.44±0.03 mm). The vitreous chamber elongation in DPA MD animals was 0.53±0.03 mm, which was not significantly different from the saline MD group. Thinning of the posterior sclera, but not the cornea, was observed in the deprived eyes of β-APN treated tree shrews, along with a tessellated appearance to the fundus. The eyes of the β-APN open animals showed no significant differences from normal. β-APN MD and DPA MD treated chickens did not develop greater myopia or vitreous chamber elongation than standard MD chickens. The selective effect of the lathyritic agents on the deprived eyes in tree shrew suggests that collagen crosslinking interacts, in the mammalian sclera, with a retinally-derived signal to regulate the elongation of the eye in myopia. © 1994 Academic Press Limited.