Extracellular space measurements using radioisotope markers (35S-sulphate and 14C-sorbitol) and ultrastructural examination of lanthanum distribution have been carried out on two vascular tissues (aorta, portal vein) exposed to normoxic and hypoxic media to see if hypoxia is associated with membrane damage. Extracellular space values did not change significantly with hypoxia of up to 60 min duration. Lanthanum remained distributed in the extracellular space in both normoxic and hypoxic specimens. These findings suggest that hypoxia may not be associated with increased vascular membrane permeability to lanthanum, as has been reported by some workers to occur with more severe metabolic deprivation.