Bacterial colonies split implanted membranes that are exposed to oral biologic fluids as a consequence of dehiscence. The clinical and histologic behavior of 14 implanted polyurethane membranes was observed during the period of exposure to oral fluids for 2, 3, 4, and 6 weeks and without dehiscence (after 8 weeks). Statistical analysis indicated that the decrease in the number of neutrophils after 5 weeks, associated with the increase in the number of activated fibroblasts, cellular debris, giant cells, and aggression of bacteria, was statistically significant (from P .05 and P .01 for activated fibroblasts to P .005 and P .001 for neutrophilic cells). The increase in bacterial passage through the polyurethane membranes and in the number of giant cells and cellular debris after 8 weeks represents late dissolution of the membranes; the progressive increase of activated fibroblasts is significant because the longer the membrane resists, the better the cells can grow and give way to the process of tissue regeneration. Palabras clave: nonresorbable membranes, oral microbiology, oral pathology