PURPOSE. To investigate the source and protein content of sub-retinal fluid in self-forming experimental serous retinal detachments. METHODS. Detachments were induced in Dutch rabbit eyes using rose bengal photosensitization to cause choriocapillaris injury and thrombosis. Serous detachments formed spontaneously within the next 24 h. Subretinal fluid was withdrawn 2, 8 and 24 hrs after photosensitization, and was analyzed for osmolality and albumin content by gel electrophoresis. RESULTS. The albumin concentration in the subretinal fluid of light-induced detachments was 68% of serum level at 3 h after light damage, and rose close to serum level by 24 h. The osmolality of subretinal fluid 24 h after light damage was essentially the same as serum and vitreous fluid. CONCLUSIONS. The subretinal protein and fluid in light-induced detachments in the central retina of the rabbit must come from the choroid, since there are no intrinsic retinal blood vessels in that region of the fundus. These data demonstrate that serous retinal detachments can form from choroidal fluid.