Changes in volume of intestinal brush border membrane vesicles of the European eel Anguilla anguilla were measured as vesicles were exposed to media with different osmotic pressures. Preparing the vesicles in media of low osmotic pressure allowed the effects of a small hydrostatic pressure to become a significant factor in the osmotic equilibration. By applying LaPlace's law to relate pressure and volume and assuming a linear relation between membrane tension and area expansion, we estimate an initial membrane tension at 4.02 x 10(-5) N cm(-1) and an area compressibility elastic modulus at 0. 87 x 10(-3) N cm(-1). The elastic modulus estimate falls in the low range of values reported for membranes from other tissues in other species. This lower modulus quantitatively accounts for why eel intestinal vesicles show measurable changes in volume in hypotonic media while rabbit kidney vesicles do not.