It is well known that large amounts of fluid are transported across the wall of mammalian kidney tubules, particularly across that of the proximal tubule. Flows near 30 nI/em2 . sec are frequently observed in the rat pruximal tubule, which signify flow speeds of 0.3 gm/sec. Representative luminal diameter ranges from 35 to 45 jim. Since the cell height is 7.5 jim UI, the latter should be crossed in some 25s by the absorptive flow, and if water flows through the cells, the cell water content must be renewed 2.5 times per mi It follows that proper understanding of the pathways and mechanisms of water and salt absorption and volume and salt regulation in these tubules must he able to discern volume changes in such small cells with enough space and time resolution. Mammalian osniolality is near 300 mOsm/liter. The mammalian proximal tubule transports fluids which are within a few mOsm of this osmolality. As an example, let us assume that we want to detect the effects of sudden application of an osmolality difference of 10 mOsm, which is reasonable value. This osmolality difference is expected to produce a cell volume change of 3%, that is, a diameter change of 1.5%, and the cell height change or diameter change expected would be of only t).45 jim in a few seconds. Therefore, precision better than one tenth of these fignrcs is needed if one is to detect the initial change in the time course of volume changes produced by such osmolality difference. These requirements in precision of time and space measurements have limited measurements of epithelial cell water osmotic pcrmeabilities and volume regulation until recently 2]. Thus the accuracy of single measurements by light microscopy, even if the image is greatly amplified, is limited by the resolving power (RP) of the objective in use: RP =' A/2.NA, where A is the light wave length (for green light A 0.55 jim) and NA is the objective numerical aperture (NA 0.65 for 40x objective). Thus, RP is about 0.4 jim and therefore a single measurement cannot provide the required precision. Despite these difficulties some solutions have been put forward.