Changes in the size, shape, and structure of the isolated tectorial membrane (TM) of the mouse were measured in response to isosmotic changes in the ionic composition of the bathing solution. Substitution of artificial perilymph (AP) for artificial endolymph (AE) caused a small (approximately 1%) shrinkage of the TM's thickness. This substitution alters not only the predominate cation (from K+ to Na+) but also the Ca2+ concentration (from 20 mumol/l to 2 mmol/l). When the predominate cation was changed from K+ to Na+, while holding Ca2+ concentration constant, results depended on Ca2+ concentration: there was a small (approximately 1%) swelling for 20 mumol/l Ca2+, larger (approximately 14%) swelling for lower (< 7 mumol/l) concentrations of Ca2+, and little response for 2 mmol/l Ca2+ or for solutions containing the Ca2+ chelator EGTA. Addition of Ca2+ while holding the predominate cation constant caused shrinkage of the TM; both removal of Ca2+ and addition of the Ca2+ chelator EGTA caused swelling. Swelling responses were largely reversible if the magnitude of the swelling was small. Responses greater than a few percent were only partially reversible and caused long-lasting changes. Changes in ionic composition of the bath affected not only the thickness of the TM but also its other dimensions. Solution changes that increase TM thickness tend to cause radial shearing motions of the surfaces of the TM, which are accompanied by small decreases in width. Little change in length was observed. Although the responses were non-isotropic, increases in thickness were highly correlated with increases in volume. Swelling of the TM was also accompanied by a reduction in prominence of its radially oriented fibrillar structure. These results for the isolated TM of the mouse are qualitatively similar to those obtained previously for the isolated chick TM (Freeman et al., 1994) but different from those obtained for the in vitro mouse TM (Kronester-Frei, 1979a).