The exact mechanism of the increased urinary excretion of albumin, a common feature of all chronic kidney diseases, is not known. Furthermore, there is a controversy both regarding the amount of albumin normally filtered by the glomeruli as well as tubular handling of albumin. Two completely different theories have been discussed in the literature: The classical one implying that the glomerular capillaries are almost impermeable to albumin and an alternative model suggesting that large amounts of albumin are normally filtered and thereafter brought back to systemic circulation by a specific tubular retrieval mechanism. Accordingly, the reported values for the sieving coefficient of albumin (filtrate to plasma concentration ratio) are in the range of 0.0006 -0.03. We present a new approach to measure the sieving coefficient of several endogenous plasma proteins in intact rats using mass spectrometry (MS). Briefly, the tubular reabsorption was completely inhibited using maleic acid and the urine (U) and plasma (P) were sampled in periods of 20 minutes. Proteins in samples of U and P were then identified and quantified using label-free MS and selected reaction monitoring (SRM). The individual protein sizes were determined by size exclusion HPLC and the pI determined by offgel isoelectric focusing. Preliminary results have given us a sieving coefficient (U/P) for albumin of 0.00053 in adult rats (n=6, average GFR = 1,022ml/min) confirming that the normal glomerular barrier is almost completely impermeable to albumin.