Previous studies have demonstrated that tissue hypoxia induces free radical generation and lipid peroxidation in the cerebral cortex of the fetus and newborn animals. The present study tests the hypothesis that hypoxia results in DNA modification through the generation of free radicals in the cerebral cortex of fetal guinea pigs. Structural alteration of DNA was determined by measuring the concentration of 8-hydroxy-guanosine, an index of free radical - induced DNA modification, using high performance liquid chromatography (HPLC). Twelve pregnant guinea pigs were exposed to 21% or 7% oxygen for 60 min. Brain tissue hypoxia in the fetus was documented biochemically by decreased levels of ATP and phosphocreatine. Cerebral cortical extracts from 6 normoxic and 6 hypoxic fetuses were prepared in 0.2 M perchloric acid for HPLC analyses using a reverse phase C18 column. A two phase gradient elution with 50 mM KH2PO4 and 60% methanol was used at a flow rate of 1.5 ml /min. Concentrations were determined by comparison with an 8-OH-guanosine standard, using peak height ratios. The concentration of 8-OH-guanosine in the normoxic and hypoxic tissues were 2.48 ± 0.46 nmoles/g tissue and 26.59 ± 4.71 nmoles/g tissue, respectively. The presence of 8-OH-guanosine in the normoxic brain suggests a basal modification of DNA molecule. The ten fold increase in 8-OH-guanosine in the hypoxic tissue (p<0.001) indicates a significant alteration in the DNA molecule due to free radical generation during hypoxia. We conclude that modification of guanosine is a potential mechanism of altered gene expression following hypoxia. We speculate that hypoxia-induced modification of guanosine results in altered translation primarily due to modification of AUG, the initiator codon. (Funded by NIH-HD-20337, MOD #6-FY94-0135, UCPR)