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Daily intake of low-dose aspirin after 12 weeks of gestation is currently recommended as a preventative intervention in pregnancies in high risk of developing preeclampsia. This recommendation is based on epidemiological evidence, whereas experimental studies investigating the exact mechanisms of aspirin action during pregnancy are lacking. We previously showed that treating pregnant rats with a synthetic mimetic of unmethylated CpG DNA (bacterial DNA) caused preeclampsia-like characteristics such as maternal hypertension and increased cyclooxygenase (COX) expression and activity. In this study, we tested the hypothesis that daily maternal treatment with low-dose aspirin would prevent the development of maternal hypertension, reduce COX activity and thromboxane A2 (TxA2) production, and improve maternal vascular function in pregnant rats exposed to CpG ODN during gestation. Pregnant rats were treated with ODN2395 (synthetic CpG DNA) or saline (vehicle) on gestational days (GD) 14, 16, 18. Daily low-dose aspirin treatment (1.5 mg/kgBW) started on GD10 and continued throughout gestation. Pregnant rats treated with ODN2395 had greater systolic blood pressure compared to controls (120 ± 4 mmHg vs. 100 ± 5 mmHg, p = 0.03) and aspirin did not prevent this increase (p = 0.86). Aspirin prevented ODN2395-induced increases of TxB2 (TxA2 metabolite) in serum and mesenteric arteries. ODN2395 increased expression of COX-1 and COX-2 in mesenteric and uterine arteries and aspirin abolished these effects. Aspirin reduced contractile responses to phenylephrine and U46619 (TxA2 mimetic) in mesenteric arteries from control rats but not from ODN2395-treated rats. In conclusion, treatment with low-dose aspirin reduced systemic and vascular COX expression and activity but did not prevent the development of maternal hypertension induced by exposure to unmethylated CpG DNA (bacterial DNA).