The mechanism by which the osmotic pressure increases in tubular fluid along the descending limb of the loop of Henle was examined in Psammomys undergoing salt diuresis. In two series of experiments, micropuncture samples were collected either from proximal and distal convolutions at the surface of the cortex, or from loops of Henle and collecting ducts at the surface of the extrarenal part of the papilla. Inulin-(3)H, urea-(14)C, Na(+), and K(+) concentrations, as well as osmotic pressure, were determined in all micropuncture samples.Net movements of water along the descending and ascending limbs of the loop could not be deduced by comparing inulin data obtained from convoluted tubules and from loops of Henle, since there appeared to be a large difference in the filtration rate of the superficial glomeruli (9 nl/min) and the deep ones (21.4 nl/min) under the conditions of these experiments. The results indicate that no large net movement of water occurred along the loop since a) only 23% of the filtrate was reabsorbed along the loop of Henle (including pars recta) of superficial nephrons despite the fact that all these loops reached markedly hypertonic regions; b) there was no positive correlation between (F/P)(In) in early distal samples and the simultaneous osmotic pressure of the urine; c) when (F/P)(In) and (F/P)(Osm) in loop samples were correlated, the increase in inulin concentration accounted only for 15% of the increase in osmotic pressure. Therefore, 85% of the concentrating process taking place along the descending limb must have resulted from net addition of solutes; this was directly supported by Na(+) and K(+) measurements in the loop samples, which showed that, at the tip of the loops, the Na(+) and K(+) flow rates were correlated to the osmotic pressure. Moreover, since the load of Na(+) urea flow rate in superficial early distal tubules was constant and independent of the urinary osmotic pressure, it is suggested that a medullary recycling of both ions occurred between ascending and descending limbs.Urea-(14)C concentration in the loop samples indicates a net addition of urea into the descending limb; the mean and K(+) delivered to the distal superficial tubules was 4.18 times its filtration rate, suggesting a recycling of urea from collecting ducts to Henle's loops. The permeability properties of the wall of the thin descending limb are discussed in relation to the obtained results.