Generalized capillary dysfunction is a morbid element in the metabolic syndrome, and it is likely involved in its complications. We tested the hypothesis that vast amounts of serum albumin previously observed in kidneys of rats with the metabolic syndrome were caused, in part, by leakage from renal peritubular capillaries. We report herein large scale leaks of plasma fluid in peritubular capillaries of rats with the metabolic syndrome. This finding was directly demonstrated in vivo, and the presence of leftover albumin residue confirmed the leak in postmortem kidney specimens. Moreover, renal interstitial fibrosis and tubular atrophy were found in a distribution similar to the leaked renal albumin in obese rats. We suggest that there is an important link between peritubular capillary damage and interstitial fibrosis, represented as tubulointerstitial disease in the metabolic syndrome. We propose that maintenance of the peritubular microcirculation may improve renal outcomes in diabetes and the metabolic syndrome. diabetic nephropathies; capillary permeability GENERALIZED CAPILLARY DYSFUNCTION is an important pathogenic mechanism of nephropathy and other complications of uncontrolled diabetes (6, 27). One logical inference from this notion is that glomerular and peritubular capillaries are similarly damaged in diabetic nephropathy. However, the focal point of research on diabetic nephropathy has been the glomerulus (15,38,39), and the prospect of significant injury to postglomerular capillaries has received less attention. Investigators have proposed that postglomerular microvessels are damaged in diabetic nephropathy (6), and some experimental support has been advanced (2, 16), prompting the suggestion that damage to peritubular capillaries alters tubular function (16). However, peritubular vasculopathy has the potential of being far more detrimental, as compromised delivery of oxygen and nutrients is potentially lethal to renal tubules (11,19). In fact, chronic peritubular capillary insufficiency might be a major contributor to tubular death and to interstitial fibrosis (1), both common features of progressive nephropathy in the metabolic syndrome (7,8).Capillary leaks of hearts, kidneys, and retinas have been described with various degrees of precision in diabetic rats (21,31,37), affirming the view that capillary damage is at least a component, if not an important source, of diabetes complications. We recently found large quantities of rat serum albumin in kidneys of male rats with nephropathy of the metabolic syndrome (7) and hypothesized a severe leakage of blood plasma into the renal interstitium, most likely from postglomerular capillaries. Hence, we searched for postglomerular capillary leakage of macromolecules in male rats with nephropathy of the metabolic syndrome. We studied F 1 generation hybrid male rats, the product of Zucker diabetic fatty (ZDF) and spontaneous hypertensive heart failure rats (ZS rats) which prominently depict all common features of the metabolic syndrome (7). We used two-photon ...