The organisation and adherence of a clot induced experimentally in the retinal vein of a cat was studied in vitro. For this purpose a retinal vein was treated in vivo with argon laser photocoagulation in 31 eyes, and each eye was removed at periods of time varying between 3 hours and 7 days after treatment. The freshly enucleated eye was then placed in a perfusion chamber with the cornea under a contact lens, and the retinal vessels were perfused under direct microscopic visualisation. It appears that the hydrostatic pressure and flow needed to perfuse the obstructed vein has to be increased with elapsing time, and that 7 days after laser treatment the retinal vessel becomes permanently occluded.