The surface of the kidney presents a unique opportunity to study fun damental processes in the living animal. Here blood vessels and kidney tubules are accessible to direct microscopic observation. This situation has been used to elucidate kidney functions under normal and pathological conditions. A number of observations on function in the intact kidney can be made through the microscope. We can observe the flow of blood in the renal microcirculation on the cortical surface. We can identify proximal and distal tubules, and measure their widths. We can follow the flow of tubular fluid with...