Xenon photocoagulation burns in the rabbit fundus were studied angiographically with fluorescein labelled dextrans of molecular weights in the range 3000 to 150 000. Recent photocoagulation burns showed dye leakage to all molecular weights used. Angiograms 2 days after burns had been produced showed leakage of dextrans of molecular weights up to and including 70 000 but no leakage of dextran of 150 000 molecular weight. At 7 days after photocoagulation healed burns remained permeable to dextrans of molecular weight 3000 or 20 000, but in the majority of eyes such burns did not leak dextrans of over 40 000 molecular weight. The clinical significance of the selective nature of photocoagulative damage to the posterior blood ocular barrier is discussed.