In this experiment, hemoglobin was used as a mass tracer. The rat received two injections, each of 1 gm of 2 X crystallized bovine hemoglobin in ~5 ml saline, administered intraperitoneally, the first at 24 and the last at 16 hours before collecting kidney specimens. Fixative was perfused into the aorta of the rat (2% osmium tetroxide buffered at pH 7.6 with acetate-veronal). Tissue blocks were stained by placing them for 30 minutes in 1 per cent phosphotungstic acid in absolute ethanol prior to infiltration and were embedded in Epon. Thin sections were stained with lead hydroxide. Micrograph was taken at original magnification of 50,000 X with a Siemens Elmiskop I, operating at 60 or 80 kv, with a double condenser, and a 50 µ objective aperture. Digitization process: conversion from a 1200 dpi tiff, 3832 x 3260 pixels. Original image created on June 11, 1962. Original resource provided as a 3.25 x 4 inch lantern slide. Research conducted at The Rockefeller Institute of Medical Research (New York, NY).