The effect of antidiuretie hormone (ADH) on transepithelial Na +, CI-, Ca 2q" and Mg 2q: net fluxes (JNa, JCI, JMg, JCa) was investigated in isolated perfused cortical thick ascending limb segments (cTAL) of the mouse nephron, using the microperfusion technique and the electron microprobe analysis to determine the ionic composition of the collected tubular fluid. Simultaneously, the transepithelial potential difference (PDte) and the transepithelial resistance (Rte) were recorded. Prior to the flux measurements cTAL segments were perfused for one hour. During this equilibration period PDte decreased significantly from +19.9~1.6 to +14.9___1.1_mV and Rte increased from 30.6+3.5 ficm to 38.8+2.4 ftcm 2 (n=7), reflecting a decline in.L, TaC1 transport. After-ADH was added to the bath solution at 10-I~ mol.1-1, PDte increased from +14.4___1.1 to +18.0-1-1.5 mV, accompanied by a rise in JNa and Jc~from ~05+11 to 273+19 and from 216+12 to 283___21 pmol.min-.mm-(n~-7), respectively. JCa and Jl~ also increased from 0.81%0.07 t 9 1.50+0.12 and from 0.43+0711 to 0.76+0.08 pmol.min=*.mm -~ (n=7~, respectively. All these effects were fully reversible after withdrawal of the hormone. In conclusion our data indicate that ADH stimulates divalent cation transport and NaC1 transport in the cortical thick ascending limb of Henle's loop of the mouse.