The mechanism by which osmolality rises in fluid in the juxtamedullary descending limb was examined in Psammomys obesus by micropuncture of the left renal papilla. The concentration of Na, Cl, K, Mg, and phosphorus was determined by electron probe; Cl concentration by both probe and the microcoulometric technique of Ramsay et al. (J. Exp. Biol. 32: 822--829, 1955). The mean urine-to-plasma ultrafiltrate (U/P) osmolality was 5.5 +/- 0.46 (SE). In 43 samples obtained from the end-descending limb in 16 animals, mean values were: tubule fluid-to-plasma ultrafiltrate (TF/P) osmolality, 3.64 +/- 0.30; (TF/P)In, 6.96 +/- 0.52; (TF/P)Na, 3.53 +/- 0.27; (TF/P)Cl-e, 4.11 +/- 0.37 (-e is electron probe determination); and (TF/P)Cl-r, 4.25 +/- 0.37 (-r is determination by method of Ramsay). In terms of percentage of filtered ion remaining at the end-descending limb, Na was 54 +/- 3.7%, Cl-e was 61 +/- 3.6%, and Cl-r was 63 +/- 4.0%. The percentage of filtered chloride remaining averaged 80% or more in three animals, equalled or slightly exceeded 100% in six individual descending limb samples, and in all animals was a function of loop fluid osmolality (y = 8.61x + 31.9, r = 0.66, P less than 0.01). The results are most consistent with the thesis that both water extraction and transepithelial NaCl addition contribute to the rise in osmolality of fluid in the descending limb of the Psammomys.