The effect of parathyroid hormone (PTH) on transepithelial Na +, C1-, K +, Ca 2+ and Mg 2+ transport was investigated in isolated perfused cortical thick ascending limbs (cTAL) and that of human calcitonin (hCT) was tested in both cortical and medullary thick ascending limbs (mTAL) of the mouse nephron. The transepithelial ion net fluxes (Jx) were determined by electron probe analysis of the perfused and collected fluids. Simultaneously, the transepitheliat voltage (PDt~) and resistance (Rt~) were recorded. In cTAL segments, PTH and hCT significantly stimulated the reabsorption of Na § C1-, Ca z+ and Mg 2+. hCT generated a net K + secretion towards the lumen and PTH tended to exert the same effect. Neither PDte nor Rt~ were significantly altered by either PTH or hCT. However, in the post-experimental period a significant decrease in PDte was noted. Time control experiments carried out under similar conditions revealed a significant decrease in PDt~ with time, which could have masked the hormonal response. In mTAL segments, Mg 2 § and Ca 2+ transport was close to zero. hCT did not exert any detectable effect on either PDt~ or Jcl-, JN~+, JK+, JM~+ and Jca2+ in these segments. In conclusion, our data demonstrate that PTH and hCT stimulate NaC1 reabsorption as well as Mg 2+ and Ca 2+ reabsorption in the cTAL segment of the mouse. These data are in agreement with and extend data obtained in vivo in the rat.