AlphaB-crystallin is a small heat shock protein that is abundantly expressed in the mammalian kidney. The present study aimed to characterize the intrarenal expression and regulation of alphaB-crystallin, and to gain insight into the functional relevance of alphaB-crystallin in the kidney. In the normal rat kidney, alphaB-crystallin mRNA and protein expression is barely detectable in cortex, whereas both increase steeply from the outer to the inner medulla, where alphaB-crystallin constitutes 2% of total tissue protein. Primarily papillary collecting ducts and thin limbs exhibited intrapapillary alphaB-crystallin immunoreactivity. In rats subjected to sucrose diuresis, mRNA expression declined by 27% and 46% in outer and inner medulla, respectively. In MDCK cells, alphaB-crystallin mRNA and protein were significantly induced by raising the medium osmolality, an effect that was concentration-dependently blunted by JNK inhibition, but not by ERK or p38 inhibition. In addition, overexpression of alphaB-crystallin resulted improved osmotic tolerance. These results indicate that alphaB-crystallin may be regulated by papillary interstitial tonicity in a JNK-dependent process and may be important for renal medullary cells.