The maximal urinary osmolality that can be reached by the kidney is reduced with age. This may be due to impaired NaC1 transport by the medullary thick ascending limb of Henle's loop, which is part of the renal concentrating mechanism and is modulated by antidiuretic hormone (ADH). We therefore tested in vitro a possible age-related change in the transport capacity and in the response of this nephron segment to ADH in young (1-2 months) and old (20-24 months) mice. The transepithelial potential difference (Vte) was significantly higher in young mice (+ 8.5_+ 0.4 mV, n = 13) than in old ones (+6.6+0.5 mV, n = 17). Addition of 0.1 nmol.1-1 ADH to the bath solution significantly increased Vte by 5.2+0.5 mV in the young and by 3.1 +0.6 mV in the old animals. Application of dibutyryl-cAMP (0.1 mmol.1-1) did not further increase the hormonal response in both groups. The ADH-mediated increase in the corresponding equivalent short-circuit current (Isc = Vte/Rte) was twice as great in young mice as in old, indicating that the stimulation of NaC1 transport by ADH across the medullary thick ascending limb is significantly reduced with age. These results suggest that the previously reported age-related defect in the urinary concentrating ability of the kidney is partly due to a decreased response of the medullary thick ascending limb to ADH.