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1. Antidiuretic hormone (ADH) was infused into normal male rats at a rate of 60 µu./min. 100 g body wt., to maintain an effectively constant maximal circulating level. Four groups of rats were used; they were water-loaded by receiving together with the ADH, I.V. infusions of hypotonic dextrose (2·5 g/100 ml.) at different rates (1·0, 4·5, 9·0 and 12 ml./hr, respectively), over an infusion period of 4 hr.

2. Urine flow rate increased in all groups, the rate and extent of the increase being related to the volume rate of infusion. The differences in urine flow rates between the four groups were due almost entirely to increases in free water clearance, with no consistent differences in osmolal clearance between the groups. At the end of the 4 hr infusion period, osmolal clearances were closely similar in the four groups.

3. Papillary and medullary tissue solute concentrations were progressively reduced at the higher rates of infusion. The changes were due to small increases in the water content, together with a profound decrease in urea concentration and a smaller decrease in sodium concentration. However, papillary osmolality was consistently higher than urine osmolality at the three highest rates of dextrose infusion.

4. As urine flow rate increased, there was a progressive reduction in the degree of osmotic equilibration between the final urine and the papillary tip. For urea, however, the degree of equilibration remained high.

5. It is concluded that, in the rat, the rate of flow per se, along the collecting duct, is an important determinant of final urine concentration; even if there is an osmotic driving force for water re-absorption in the renal medulla, and the collecting duct walls are permeable to water, osmotic equilibration is restricted by tubular flow rate.