The purpose of the present studies was to examine, by micropuncture, the effect of salmon calcitonin on fractional sodium and solutes deliveries to the juxtamedullary end-descending limb of the rat. All animals were postprandial and thyro-parathyroidectomized Munich-Wistar rats. Group 1 (N = 8) consisted of time control water-diuretic rats; group 2 (N = 8) received synthetic salmon calcitonin (10 mU/min) intravenously while undergoing water diuresis; group 3 (N = 8) was treated as group 2 but also received calcium intravenously to prevent the calcitonin-induced fall in plasma calcium. Calcitonin, alone and with calcium, produced a marked fall in urine flow rate and a marked increase in urinary osmolality. Concomitant fractional water delivery to the end-descending limb fell significantly (28 ± 0.8 to 21 ± 1.0%,p < 0.05), while fractional solute and sodium deliveries increased significantly (36 ± 1.3 to 55 ± 2.6%,p < 0.05; 34 ± 2.0 to 48 ± 3.5%,p < 0.05, respectively). The three groups did not significantly differ in fractional water and sodium deliveries to the superficial end-accessible proximal tubule. We conclude that salmon calcitonin is antidiuretic in the rat and that it also produces an increase in fractional sodium and total solute deliveries to the end-descending limb, which we suggest is due to transepithelial sodium addition. The physiological significance of these observations to water homeostasis in vivo remains to be determined.