Sorbitol plays an important role in the osmoregulation of several renal cell types, especially the inner medullary collecting duct (IMCD) cells. Very little information is available concerning the expression of the enzymes of sorbitol metabolism (aldose reductase (AR) and sorbitol dehydrogenase (SDH)) on the RNA level under different osmotic conditions. We employed a RT-PCR-based strategy to investigate the regulation of mRNA coding for AR and SDH. For AR two primers (derived from the sequence of the rat eye lens) were chosen which amplify a 668-bp product. For SDH (considering the sequence of rat liver) three primers were chosen, amplifying a 367- and a 1, 068-bp fragment. Digestion with restriction enzymes and sequencing of the products clearly indicate that the specific mRNA of AR and SDH was amplified. By relative quantitative determination of the amplification products a more than 4-fold increase in mRNA for AR in IMCD cells was observed within 24 h after increasing the extracellular osmolarity from 600 to 900 mosm/l. Decreasing the osmolarity from 600 to 300 mosm/l resulted in a reduction in the mRNA level by 70%. The complete adaptation of the AR activity needed 3 (increasing osmolarity) and 6 days (decreasing osmolarity). Osmotically induced alterations in the levels of mRNA coding for SDH could not be observed. These results suggest that the adaptation of sorbitol synthesis occurs by a rapid regulation of transcription or stability of specific mRNA. For a complete synthesis or degradation of AR 3-6 days are necessary. Thus sorbitol synthesis in IMCD is more rapidly adapted to increasing osmolarities than to decreasing osmolarities.