Purpose: Recent studies have shown rapid and significant changes in the thickness of the retina, RPE and choroid of young chickens following termination of 2 weeks of monocular translucent occlusion . The aim of this study was to examine the outer retinal structures for morphological evidence which may elucidate the mechanisms responsible for the choroidal thickness changes.Methods: 75 broiler chicks were reared with a translucent styrene occluder over one eye from day 1 to day 15, at which time it was removed and the animals given normal visual experience for up to 6 days. The resulting structural and refractive changes were followed biometrically and ultrastructurally. Refractive status was measured in 9 groups from time 0 - 120 hours after occluder removal and later compared with morphological measures of recovery including retinal thickness, choroidal thickness and thickness of Bruch's membrane. Results: As expected, the choroid increased in thickness dramatically over the first 72 hours, then decreased again towards the thickness of the control fellow eyes. Retinal thickness, while increasing in thickness over the same period as the choroid, showed a much flatter profile with time. Bruch's membrane showed a completely different time course, remaining thinner than normal until 96 hours after occluder removal. Detailed electron microscopic images during the period of recovery show evidence for the re-establishment of the outflow to the choroid, especially as indicated by gross fluctuations in the infoldings of the basal lamina of the retinal pigment epithelium and the presence of putative fluid vacuoles during the time of maximal choroidal distension. Conclusion: The electron microscopic images during the period of recovery show evidence of a rapid increase in fluid movements across the retinal pigment epithelium and re-establishment of the outflow to the choroid. A comparison of the rates of thickness change for the various measures with the refractive error change indicates that a model of direct choroidal compensation by actively moving the retina to control refractive error requires modification. A model has been established, based on these results, which associates the changes in choroidal thickness with a passive response to fluid transfer which is controlled at the level of the retinal pigment epithelium