Anionic sites are thought to act as anionic barriers regulating passage of proteins across Bruch's membrane of the eye. The cationic electron microscope tracer. polyethyleneimine (PEI), was used to study the distribution of the anionic sites in the Bruch's membrane of the control rabbit eyes and sodium iodate injected eyes. In Bruch's membrane of the control eyes, electron dense particles are present along the both sides of the basal lamina of the RPE and choriocapillaris and along collagen and elastic fibers. In the sodium iodate treated eyes, the RPE cells are severely degenerated and anionic sites are significantly decreased in the Bruch's membrane. These changes suggest that RPE cells play an important role in maintaining the anionic sites in the Bruch's membrane.