AbstractThe purpose of this investigation was to study the hydrostatic pressure changes within and outside the proximal tubules and to study possible changes in proximal cellular volume during the occlusion period following abruption of the renal blood flow and filtration. The pressures were measured directly using the micropuncture technique. The results demonstrate that the proximal tubular wall can resist a modest positive hydrostatic pressure from either the inside or the outside of the cellular wall for a short time interval. During reabsorption in the occlusion period a positive pressure seems to be created in the lumen of the proximal tubules probably due to “interfacial tension” and a positive luminal pressure is maintained until all the fluid has disappeared.The proximal cellular volume was estimated from the cellular area of cross‐sectioned proximal tubules of snap‐frozen kidneys. The area was measured by planimetry. It was demonstrated that the cellular area is the same (within about 10%) at the various rates of transcellular transport of sodium associated with spontaneous variations of the rate of glomerular filtration, and that it remains unchanged during the whole occlusion period. An increase of about 50% in the cellular volume occurred as soon as the luminal fluid was reabsorbed, i. e. about 20 sec after interruption of the renal circulation. The cellular volume then remained unchanged for several