Atrazine is a widely used triazine herbicide for controlling broadleaf and grassy weeds. The widespread use of atrazine has resulted in the contamination of many surface and ground waters in the U.S. Our recent toxicogenomic analysis of atrazine-treated and -untreated fourth-instar larvae of the aquatic midge Chironomus tentans, using restriction fragment differential display (RFDD)-PCR, revealed various up- and down-regulated genes in atrazine-treated midges that may potentially affect respiration and oxygen transport. This study is being conducted to evaluate the oxidative stress imposed upon atrazine-treated midges and the molecular mechanisms that may elicit and/or counteract such effects. Atrazine at 1, 10, 100, and 1,000 µg/L significantly enhances the respiration of midges by 15, 18, 25, and 24% and 29, 33, 37, and 58% following 12- and 24-h exposure, respectively, compared with controls. Similarly, midges exposed to atrazine concentrations from 1 to 1,000 µg/L for 12 and 24 h consumed approximately 24 to 38%, respectively, more oxygen compared with controls. Furthermore, two up-regulated hemoglobin genes from atrazine-treated midges were isolated by screening a C. tentans cDNA library. The time- and dose-dependent expression patterns of both hemoglobin genes will be examined using northern analysis. This study is expected to provide insight into the physiological risk that atrazine imposes upon aquatic non-target organisms as well as assist in the identification of atrazine-specific biomarkers that may potentially be used to evaluate non-target effects of atrazine and aquatic environmental health.