Organic osmolytes such as betaine and glycerophosphorylcholine (GPC) are of major importance concerning volume regulation of inner and outer medullary epithelial cells. Recently we demonstrated that the intracellular betaine content in rabbit kidney cells derived from the outer medullary thick ascending limb of Henle's loop (TALH) is osmotically regulated by betaine synthesis. In this context it was our purpose to characterize the uptake of choline, a precursor of betaine and GPC. We found TALH cells to possess a specific choline transport system with a maximum velocity (Vmax) of 71 +/- 12 pmol . micro l-1 cell water . min-1 and an apparent affinity (Km) of 155 +/- 19 micromol . l-1. The uptake of choline was sodium independent and not electrogenic, but it was significantly reduced by the removement of chloride from the incubation medium. After long-term adaptation of TALH cells to a hyperosmotic medium (600 mosmol . l-1, osmolarity adjusted with NaCl or urea) a significant higher choline uptake rate was observed (Vmax: 166 +/- 9 (NaCl), 96 +/- 12 (urea) pmol . microl-1 cell water . min-1). Our results suggest that the uptake of choline is due to higher intracellular requirements of choline under hypertonic conditions. Finally, an increase in the Vmax of the choline transport system may enable sufficient synthesis of betaine and GPC.