The effects of physiological doses of human calcitonin (HCT) on renal excretion and tubular transport of water and electrolytes were investigated in hormone-deprived rats, i.e., homozygous DI Brattleboro rats with reduced levels of circulating glucagon, parathyroid hormone, and thyrocalcitonin, as these hormones are believed, together with ADH, to stimulate the same cells of the thick ascending limb. The experimental design was similar to the one used in a preceding study aimed at determining the effects of ADH in hormone-deprived rats [C. de Rouffignac et al. Am. J. Physiol. 244 (Renal Fluid Electrolyte Physiol. 13): F156-F164, 1983]. In the present experiments, HCT consistently increased the reabsorption of Mg, Ca, and K and, to a lesser extent, Na and Cl in the loop of Henle, but phosphate transport did not rise. The urinary excretion rate of Mg and Ca fell significantly. These data are very similar to the findings obtained with ADH on hormone-deprived rats. It is concluded that, in vivo, administration of HCT 1) stimulates reabsorption of Na, Cl, Mg, Ca, and K by the thick ascending limb, and 2) consistently enhances Mg and Ca reabsorption by the whole kidney by enhancing reabsorption in the loop of Henle. The similarity of the physiological responses elicited by ADH and calcitonin on the thick ascending limb supports the hypothesis of multiple hormonal control of electrolyte transport by the thick ascending limb.