PURPOSE:Chick choroidal lymphatics swell rapidly during recovery from form-deprivation occlusion, leading to the question of functional significance. To date, analysis of lymph in initial lymphatics has been problematic because of the difficulties of access. However, elemental composition can be determined using scanning electron microscope (SEM) X-ray microanalysis. This study investigated whether cryo-fixation would permit vascular fluids to be analysed in situ. METHODS:Two chicks were raised normally and seven were raised with monocular occlusion for 12 days before varying periods of normal visual experience. The eyes were rapidly frozen, fractured and X-ray spectra obtained from the lumina of lymphatic and blood vessels in a Cambridge S360 SEM. RESULTS:The elemental signatures of Na/Cl/K/P/S distinguished the two vessel types. CONCLUSIONS:These results suggest this bulk-frozen tissue technique can quantify relative changes in elemental species present in various ocular compartments in response to light-induced changes in the retina.