P reviously we demonstrated that arginine vasopressin (AVP) directly inhibits bicarbonate absorption (JHCO3, pmol/min per mm) in the medullary thick ascending limb (MTAL) of the rat. To determine whether changes in osmolality also may affect bicarbonate absorption, MTAL were studied in vitro with 25 mM HCO3- solutions. Control osmolality was 290 mosmol/kg H2O. In the absence of AVP, increasing osmolality to 560 in perfusate and bath by addition of 150 mM NaCl reduced JHCO3 from 13.7 to 4.5. With 2 x 10(-10) M AVP in the bath, adding 150 mM NaCl to perfusate and bath reduced JHCO3 from 6.9 to 0.6, while adding NaCl to the bath alone reduced JHCO3 from 7.1 to 0.5. Adding 150 mM NaCl to perfusate and bath caused a similar inhibition of JHCO3 in MTAL perfused with furosemide to inhibit net NaCl absorption. In the presence of AVP, adding 600 mM urea to perfusate and bath inhibited JHCO3 by 55%; adding 300 or 600 mM mannitol to perfusate and bath inhibited JHCO3 by 75%. The effects on JHCO3 were reversible and dissociable from changes in transepithelial voltage. Conclusions: (1) osmolality is a factor capable of regulating renal tubule bicarbonate absorption; (2) hypertonicity produced with NaCl, urea, or mannitol markedly inhibits bicarbonate absorption in the MTAL; (3) this inhibition occurs independent of, and is additive to, inhibition by vasopressin. Hypertonicity may shift TAL HCO3- absorption from medulla to cortex, thereby limiting delivery of bicarbonate to the medullary interstitium during antidiuresis.