The normal filtered load of albumin at the rat glomerulus was estimated to be 18 mg/24 h × g kidney weight by means of micropuncture studies. Total nephron fractional reabsorption was 91%. The proximal convoluted tubule accounted for 72% of total reabsorption followed by an additional 12% in the loop of Henle. Only minimal protein uptake occurred in more distal parts of the nephron. Na maleate, given at a dosage of 2 mM/kg body weight 2 h prior to micropuncture, inhibited tubular albumin uptake in the proximal convoluted tubule. As in controls there was no measurable protein uptake beyond the loop of Henle. These results indicated that tubular reabsorption of albumin was virtually completed by the early distal convoluted tubule. Maleate was found to be a potent inhibitor of protein transport in the proximal convoluted tubule resulting in a fractional excretion of albumin of 50%