The thick ascending loop of Henle (TALH) is exposed to high osmotic stress, which is particularly due to high sodium and chloride reabsorption and very low water permeability of the luminal membrane. Therefore, the volume regulation of TALH cells, derived from the TALH loop of rabbit kidneys, was analyzed. The volume was determined by impedance measurements. TALH cells, which were adapted to different osmolarities (300 and 600 mosm/l), showed no significant differences in their cell volume. Therefore, a complete volume regulation could be supposed. An increase in extracellular osmolarity from 300 to 600 mosm/l (osmolarity adjusted by addition of 150 m M NaCl) immediately led to a reduction in the cell volume by 37 ± 6% (n = 6). A regulatory volume increase (RVI) was not observed within 10 min but after 24 h. Conversely, a sudden cell swelling by 44 ± 5% (n = 4) was detected within 20 s following an extracellular hypoosmotic challenge (from 600 to 300 mosm/l). The subsequent volume regulatory decrease (RVD) required a period of 7 days. Specific inhibitors of important ion transporters had no effect on volume regulation. Thus, changes in the ion conductivity do not seem to influence the processes of RVI and RVD. Conversely, the intracellular content of the organic osmolytes, sorbitol, inositol, betaine, and glycerophosphorylcholine, changed in the course of RVI and RVD. These results provide evidence that TALH cells are capable of maintaining their volume despite large extracellular osmotic changes. RVI and RVD are mainly regulated by changes in the intracellular content of organic osmolytes within 1 and 7 days.