Previous studies have indicated that hemorrhage may predispose the lung to the respiratory distress symdrome. Gene expression profiling with oligonucleotide microarrays was used to evaluate the genetic responses of the lung to hemorrhage. Conscious rats, chronically instrumented with a catheter and telemetry device to record blood pressure, heart rate, and temperature, had 40% of their estimated blood volume removed at a rate of 1 ml per minute over 7-10 minutes. Groups of three or more rats were euthanized at 1, 3, 6, 16, 24, 48, or 72 hours following hemorrhage. Two additional groups were unmanipulated controls and instrumented animals with sham-hemorrhage. Total RNA was isolated from lung, reverse-transcribed to cDNA, fluorescently labeled, and hybridized to oligonucleotide micorarrays probing 5671 rat genes. Following hemorrhage, statistically detectable alteration of expression was seen in approximately 0.8% of the genes at some time during the 72-hour test period (vs sham hemorrhage) as determined by false-discovery rate statistics in the Statistical Analysis of Microarrays program. A subset was confirmed by reverse-transcriptase polymerase chain reaction analysis. Hemorrhage influenced genes that regulate intracellular signaling and structure, growth factors, and hormonal receptors. There also appeared to be increased expression of genes that may mediate sequestration of neutrophils and mononuclear cells from the circulation. This hemorrhage model, though producing severe hemodynamic alterations, avoided mortality and histologic evidence of lung damage, a feature intended to help insure reliable evaluation of gene expression. These results indicate that gene expression profiling with microarrays provides a new tool for exploring the response of a tissue to systemic blood loss.