Microfluorimetric and quantitative cytoenzymatic techniques were employed to examine regional variations in endothelial macromolecular uptake as related to inner mural intermediary metabolism in aortas from normocholesterolemic rabbits. Concomitant reductions in luminal fluorescein isothiocyanate-conjugated bovine serum albumin (FITCBSA) accumulation and succinate (SDH), lactate (LDH) and glucose-6-phosphate (G-6-PDH) dehydrogenase activities were evidenced from ascending to upper abdominal segments. Further diminution of FITCBSA accumulation was observed in the lower abdominal aorta, whereas there was a corresponding elevation of enzyme activities. Highly significant but not exceedingly close correlations were obtained between luminal FITCBSA uptake and inner mural SDH and LDH activities. However, much of the associated variability was attributable to the lower abdominal segment, where there was a microscopically-discernible augmentation in adventitial surface FITCBSA accumulation. The overall data provide direct in vivo support for the concept that endothelial macromolecular transport is coupled to mural oxidative demands, but also indicate that luminal metabolism-permeability relationships are influenced by factors such as extensiveness of the vasoral network and wall thickness. Details of the metabolism-permeability coupling hypothesis and observations implicating metabolic events as basic causative factors underlying vascular pathogenesis are discussed.