premise that the hairpin-like loop of Henle acts as a countercurrent multiplier system. The original experi- mental observations in support of this concept were those of Wirz, Hargitay and Kuhn (5) who concluded from freezing-point studies of slices from concentrating rat kidneys that the osmolality was identical for all adjacent tubular structures at any level in the kidney, and that there was a steadily increasing osmotic gradient from the cortex to the tip of the papilla. The cortex itself was found to be isosmotic with plasma. Subsequently, Wirz (6) showed that the medullary blood took part in this mechanism by demonstrating that blood from the vasa recta at the tip of the hamster papilla was as hyper- osmotic as the urine. Although not invalidating the concept of an increasing osmotic gradient from cortex to papilla, the recent analyses of distal tubular fluid by Wirz (3) and those reported in this paper demonstrate that all tubular fluid does not have exactly the same osmolality at a given level in the kidney, and suggests to us that post-mortem diffusion probably accounted
for this aspect of the results of Wirz, Hargitay and Kuhn (5).