In hypertonic bath, anuran bladder exhibits a reversible increase in osmotic permeability. This response to serosal hypertonicity (the SH response) has been considered to be partly due to epithelial cell shrinkage with a consequent increase in intracellular cAMP concentration, a known mediator of the ADH-water response. We have shown that the SH response in Rana temporaria bladder may be modulated (Hanna-Mitchell & Gebruers 2001, 2002). These findings indicate that specific biochemical events are involved in the SH response. Results reported here are from a combined functional and morphometric study of paired hemibladders from humanely killed male Rana temporaria; one hemibladder was fixed following a five-minute exposure to SH (SH+5 group) and the corresponding hemibladder was fixed following a 30-minute exposure to SH (SH+30 group). Following fixation in hypertonic medium, bladders were processed both for TEM imaging and for light microscopy. Using stereological methods, granular cell volume was calculated for each of the groups; granular cells are functionally analogous to the principal cells of the mammalian nephron. The SH+5 group exhibited a significant (p<0.05) mean water loss of 3.13 ±0.6 mg.min-1 compared with 0.5±0.4 mg.min-1 in control isotonic bath, over a similar time interval. SH+30 group hemibladders showed a significant (p0.05. Granular cells in both groups exhibited non-dilated junctional complexes, microvilli and a greatly expanded intercellular space, consistent with transepithelial water movement. Further examination of a possible role for granular cell volume in the hydro-osmotic response is warranted. Paired Student t-test was used for statistical analysis and values are expressed as mean?SEM.