The central importance of xanthine dehydrogenase (XDH) and xanthine oxidase (XO) in the pathobiochemistry of a number of clinical disorders underscores the need for a comprehensive understanding of the regulation of their expression. This study was undertaken to examine the effects of cytokines on XDH/ XO activity and gene expression in pulmonary endothelial cells. The results indicate that IFN-'y is a potent inducer of XDH/XO activity in rat lung endothelial cells derived from both the microvasculature (LMVC) and the pulmonary artery.In contrast, interferon-affi, tumor necrosis factor-a, interleukin-i or -6, lipopolysaccharide and phorbol myristate acetate have no demonstrable effect. The increase in XDH/XO activity requires new protein synthesis. By Northern analysis, IFN-"y markedly increases the level of the 5.0-kb XDH/XO mRNA in LMVC. The increase is due, in part, to increased transcription rate of the XDH/XO gene. Transcriptional activation does not require new protein synthesis. The physiologic relevance of these observations was evaluated by administering IFN-'y to rats. Intraperitoneal administration leads to an increased XDH/XO activity and XDH/XO mRNA level in rat lungs. In sum, IFN-'y is a potent and biologically relevant inducer of XDH/XO expression; the major site of upregulation occurs at the transcriptional level. (J. Clin. Invest. 1992. 89:197-202.)