Because protein transport between blood and artery wall is important in atherogenesis, we have measured the uptake of fluorescently labelled albumin around the aorto-renal branch, an important site for lesions. Tracer concentrations in artery wall sections were quantified using digital fluorescence microscopy. Short-term experiments indicated endothelial permeability, while longer ones indicated the steady-state distribution of native proteins within the wall. In sexually immature rabbits (65–75 days), the permeability of the aorta was greater downstream of the renal ostium than upstream (p < 0.004). However, in mature rabbits (120–156 days), the permeability was greater, on average, at the upstream site compared to downstream. This change in pattern of uptake around the renal ostium with age appears to be due to increased uptake at the upstream site in mature animals, which was over fourfold greater than that measured at the equivalent site in immature animals (p < 0.01). In the renal artery itself, permeability was low and age independent. In mature animals, endothelial permeability did not correlate with steady-state wall concentrations, suggesting that the latter may be more dependent upon the rate of exit of protein and/or available space in the wall, rather than the rate of entry. The age-dependent changes in the spatial pattern of permeability correspond to the different distributions of spontaneous lipid accumulation in immature and mature vessels of both rabbits and humans, implying that permeability of the wall is important in atherogenesis.