in cross section, and by their relatively low cells. Henle's loop and the collecting tubules were recognized in the medullary ray and in the medulla; in the papilla, collecting tubules only were found. When the various parts of the kidney had been identified in the stained section, they were dissected out of the adjacent unstained section with microscalpels under a dissection microscope (magnification x40).Separate dissection of Henle's loops and collecting tubules was rarely possible because of their intimate anatomical relationship.The dissected specimens were weighed on a quartz fiber fish-pole balance (sensitivity 0.4 m,gg; useful range 10 to 100 mpeg) (2). A manipulator made from a microscope mechanical stage was used to load and unload the balance pan. A horizontal microscope fitted with a vertical fine adjustment was used to read the displacement of the pan. Under the dissection microscope, each of the weighed specimens was transferred to the bottom of micro test tubes (inside diameter, 3 mm) by a fine glass needle. Both dissection and weighing were done in a room maintained at low humidity and relatively constant temperature. The specimens were assayed to determine alkaline phosphatase activity by adding 3 .l of substrate reagent [0.5M 2-amino-2-methylpropanol-1 (pH 10.0), 8 mmole/lit of p-nitrophenylphosphate, 2 mmole/lit of 1M MgCl2, and 0.05-percent bovine serum albumin (3)]. After 1 hour of incubation at 37?C, the reaction was stopped and color was developed by adding 50 t1 of 0.1N NaOH. The optical density of the solution was read in Lowry-Bessey microcuvettes (4) at 410 m.u in the Beckman DU spectrophotometer, and the results were expressed in moles of substrate split per kilogram (dry weight) per hour.Typical data for individual kidneys of dog, rat, and man are shown in Table 1. The results within each species were consistent, although somewhat different distributions were found in the tubules of each species. The experimental reproducibility for renal homogenates, expressed as standard deviation, was found to be +0.8 moles split per kilogram
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