Hypoxic pulmonary vasoconstriction (HPV) is an adaptive mechanism that normally helps to optimise ventilation-perfusion matching in the lung by diverting blood from poorly ventilated areas. Although we know that the central mechanism (or mechanisms) of HPV resides in the smooth muscle cells of the small pulmonary arteries, there is still controversy concerning the O2 sensing mechanism and its transduction pathways. There are three competing hypotheses: the Redox hypothesis suggests that hypoxia causes a reduction in mitochondrial ROS production and a more reduced cytosolic redox state, which inhibits voltage gated K+ channels and causes depolarisation and Ca2+ entry. The ROS hypothesis however suggests that mitochondrial ROS production is increased, and that ROS activate Ca2+ release from ryanodine-sensitive stores, consequent store operated Ca2+ entry, and also importantly Rho kinase-mediated Ca2+ sensitisation. The third hypothesis does not depend on ROS, but instead invokes a change in energy state as reflected by an increase in cytosolic [AMP], which activates AMP-activated kinase. This, possibly in conjunction with elevated NADH, is thought to lead to activation of ryanodine receptors via cyclic ADP ribose. We and others have considerable evidence to support the ROS hypothesis, although as will be discussed this does not necessarily conflict with a role for AMP kinase. This includes measurements of ROS (though this is in itself highly contentious) , the suppressing effect of antioxidants both exogenously applied and by over-expression of intracellular antioxidant mechanisms, and in particular the effects of exogenously applied ROS in the form of superoxide and peroxide, both of which cause constriction in pulmonary artery. Whilst the potential role of ROS in the hypoxia-induced elevation of intracellular Ca2+ will be discussed, the major focus of the this talk will concern the role of ROS in the activation of Rho kinase mediated Ca2+ sensitisation, as this is of particular importance in sustained HPV and many models of pulmonary hypertension. In summary, we believe that the balance of the evidence favours the ROS hypothesis of HPV, although as with the other hypotheses many factors remain unclear.