Ultrastructural studies were made of 7 glutaraldehyde-treated porcine valvular heterografts. Five prosthesis had been preserved for a period of 24 months. The remaining two, presented patched areas of different appearance and were eliminated at the routine control studies. Microscopic and electron transmission microscopic observations demonstrated disruption of the endothelial cell layer with preservation of the architecture in the remaining layers. This is consistent with previous works. The other two valves presented lipid degeneration of the spongiosa in one and diffuse fibrous degeneration in the other. This work demonstrates that valves submitted to prolonged storage periods do not undergo further alterations than those recently preserved. It also demonstrates that the pigs own pathology may cause significant structural modifications of the tissue which are only detected with rigorous examination using magnifying optical techniques.