Introduction: Prenatal hypoxia remains a major concern in obstetric practice today. The fetal defence to hypoxia includes redistribution of the cardiac output, away from peripheral and towards essential circulations, such as those perfusing the brain (Cohn et al. AJOG 120:817, 1974). The physiology underlying this response is well characterised and involves chemoreflex and endocrine responses (Giussani et al. Fet Mat Med Rev 6:17, 1994). More recently, local factors such as nitric oxide (NO) and reactive oxygen species (ROS), and the interaction between them, have been shown to play a role. We have previously reported that allopurinol, a xanthine oxidase inhibitor, attenuates the increase in femoral vascular resistance in response to acute hypoxia in the late gestation sheep fetus (Hansell et al. Reprod Sci 15(2) Suppl. A615, 2008). In this study, we have investigated whether this depression is due to allopurinol increasing the bioavailability of NO. Methods: Under halothane anaesthesia, 6 fetal sheep were instrumented at 0.8 gestation with catheters and a femoral Transonic probe. Five days later, all animals were subjected to 2 h normoxia, 0.5 h hypoxia (reduction in mat FiO2) and 1 h recovery, either following maternal i.v. treatment with vehicle, allopurinol (150 mg.kg-1 over 30 min) or allopurinol during the NO clamp. The latter is a technique that blocks de novo synthesis of NO while maintaining basal cardiovascular function in vivo (Gardner & Giussani, Circulation 108:331, 2003). The timing of the hypoxic challenge coincided with peak concentrations in fetal sheep plasma of oxypurinol (the active metabolite). Results: During hypoxia, a similar fall in fetal PaO2 occurred in all fetuses (vehicle: 20.2±0.9 to 9.2±0.5; allopurinol: 21.1±1.3 to 9.9±0.7; allopurinol+NO clamp: 23.8±0.9 to 10.2±0.9 mmHg). Maternal treatment with allopurinol attenuated the fetal femoral constrictor response to acute hypoxia, an effect which was prevented by combined treatment with allopurinol and NO blockade (Fig. 1). Conclusion: Maternal treatment with allopurinol attenuates the femoral constrictor response to acute hypoxia by increasing the bioavailability of NO in the late gestation ovine fetus.