Summary The results of interference-microscopical thickness measurements of isolated nephrogenic basement membranes are reported and discussed. Tubular and glomerular basement membranes were examined by the shearing technique with total image splitting, the kidneys used having obtained from pigs as well as from human beings. It was not possible to determine a statistically significant difference in thickness between tubular and glomerular basement membranes obtained from pigs. Human glomerular membranes were found to be much thicker in long-time diabetics than in normal subjects. Also, glomerular membranes of young persons are thinner than the membranes of adults.