The aim of this study was to evaluate the effects of Escherichia coli-derived lipopolysaccharide on guinea pig kidney by measuring the energy charge ratio and 3-nitrotyrosine levels. In addition the possible protective role of melatonin against lipopolysaccharide-mediated peroxynitrite formation and energy depletion of kidney was determined. Guinea pigs were either pretreated with melatonin or saline (for the control) followed by intraperitoneal administration of E. coli. Six hours after the administration of E. coli, guinea pig kidney ATP, ADP, AMP and 3-nitrotyrosine levels were measured by reverse-phase high performance liquid chromatography. There was a significant increase in the formation of 3-nitrotyrosine and decrease in energy charge in the endotoxin-induced group. However melatonin administration prevented 3-nitrotyrosine formation while failing to prevent or restore changes in the energy charge ratio of the kidney. Copyright (c) 2004 John Wiley & Sons, Ltd.