Central to the assumption that glomerular capillary pressure (Pgo) can be equated with the sum of arterial oncotic pressure (z~t) and the pressure in a blocked proximal tubule ("stop flow" pressure, PsO is that filtration ceases in the blocked nephron. Should filtration not cease, but continue at a rate equal to tubular reabsorption between the block and the glomerulus, P~f, for a given Pg~, will depend on the distance between block and glomerulus. This would have serious consequences for the interpretation of P~f, particularly in respect of its frequent use in analysis of the tubuloglomerular feedback (TGF) mechanism. Experiments were performed in anaesthetized Wistar rats to examine whether a length dependency of P~ exists and, if so, to what extent this relationship alters during maximal TGF stimulation by loop of Henle perfusion. A length dependency of P~f existed both in the absence and presence of loop flow. The regression coefficients were significantly different from 0 and from each other. Pgo cannot thus be equated with the sum of P~f and zr,~t. The length dependent error in P~f makes it unsuitable for the quantitative analysis of TGF and glomerular haemodynamics.