The reabsorption of endogenous albumin in the proximal tubules of the normal Munich-Wistar rat has been studied using immunoperoxidase techniques. The lumina of the superficial proximal tubules were found to remain open after in situ fixation, allowing good penetration of the conjugates. Labeling typical of an endocytotic process was observed only in the proximal tubules. This fact suggests that the glomerular filtration barrier is not an absolute one for denogenous albumin.