The proximal tubular fluid albumin concentration of rats with immunologically induced nephrotic syndrome rose from 0.60 +/- 0.11 (SEM) mg/100 ml to 7.42 to 0.87 mg/100 ml. The glomerular filtration rate (GFR) was slightly depressed (P less than 0.001) and fractional water absorption unchanged (P greater than 0.45). Urinary albumin excretion rose from 0.18 +/- 0.03 (SEM) mg/100 g of body wt/24 hr to 78.5 +/- 39.5 mg/100 g/100 g of body wt/24 hr (P less than 0.001). The concentration of albumin in glomerular filtrate, calculated as the product of proximal tubular albumin concentration and fractional water absorption (4.53 +/- 0.60 mg [SEM]/100 ml), equalled that calculated from urine albumin concentration and whole kidney water absorption (4.14 +/- 0.84 mg/100 ml, P greater than 0.30). The data are interpreted to reflect a significant increase in filtered albumin and only a modest (if any) tubular albumin absorption in this study. Thus, the concept that tubular absorption plays a major role in the catabolism of albumin and the genesis of hypoalbuminemia in the nephrotic syndrome could not be confirmed.