Collagen cross-linking induced by lysyl oxidase has been implicated in liver and lung fibrosis. To define the role of this process in kidney fibrosis, we investigated the renal expression of lysyl oxidase and the content in collagen cross-links at various stages of chronic Adriamycin nephropathy in Sprague-Dawley rats. Lysyl oxidase expression was determined by RT-PCR; collagen pyridinium residues, indicating lysyl oxidase induced cross-links, were evaluated by HPLC. These parameters followed a synergic albeit asynchronous outcome: (a) lysyl oxidase mRNA levels in total kidney, glomeruli and medulla from Adriamycin-treated rats increased up to 3 times compared to controls between week 8 and 12, then returning within the normal range; (b) the pyridinium residue content did not show any significant difference between Adriamycin-treated and control rats, until diffuse interstitial fibrosis developed (16 weeks), showing at this time a 2- to 3-fold increment. Lysyl oxidase was expressed by several renal cell lines and in tubular-epithelial cells it was up-regulated in vitro by TGF beta-1, a recognized fibrogenetic factor in Adriamycin nephropathy. Our observations demonstrated that an increased expression of lysyl oxidase in the kidney precedes the development of diffuse fibrotic lesions and that, at this stage, collagenic structures contain highly cross-linked components, the final product of lysyl oxidase activity. The evidence of lysyl oxidase up-regulation in tubular epithelial cells by the same factor implicated in Adriamycin toxicity in the kidney suggests a common pathogenetic mechanism. Collagen cross-link formation by lysyl oxidase may be implicated in the pathogenesis of irreversible, fibrotic renal lesions.