Hypoxic pulmonary vasoconstriction (HPV) is a lung-specific mechanism which serves to match pulmonary ventilation and perfusion under physiological conditions, thus optimising gas exchange. However, with global pulmonary hypoxia, e.g., during chronic obstructive pulmonary disease, HPV causes pulmonary hypertension, vascular remodelling and heart failure. The cause of HPV is most likely multifactorial and Ca²⁺ influx, Ca²⁺ release and Ca²⁺ sensitisation have been implicated with its development. Recently, Ca²⁺ extrusion via Na⁺-Ca²⁺ exchange (NCX) was reported to be abolished in single cells under hypoxic conditions, leading Wang et al. (2000) to propose that HPV might be due to an inhibition of NCX by hypoxia. We have therefore investigated whether NXC plays a role in HPV in intact rat small pulmonary arteries (PA). Isolated PA (200-600 μm i.d.) from humanely killed rats were mounted on an isometric myograph, preconstricted with PGF_2α and exposed to hypoxia (0% oxygen) for 40 min. This evoked a biphasic contractile response, which consisted of a transient contraction (phase 1) followed by a progressively increasing sustained contraction (phase 2). Application of inhibitors of the NCX and Na⁺ substitutes were used to assess the role of NCX activity during HPV. PA were pretreated with the Ca²⁺ antagonist diltiazem (10 * 10^-6M) in order to prevent effects on contraction which might be caused by membrane depolarisation. All values represent mean contraction relative to high K⁺ contraction ± SEM in control and in the presence of drugs, respectively. Student’s T-Test and the Mann-Whitney Rank Sum Test were used to determine statistical significance. The NCX inhibitor KB-R7943 (30 *10 ^-6 M) almost abolished phase 1 (67 ± 5% vs. 17 ± 5%) and significantly reduced phase 2 (33 ± 5% vs. 20 ± 6%), whereas amiodarone (50 *10^-6 M), which also inhibits NCX, significantly blocked phase 1 (38 ± 2% vs. 25 ± 3%), but not phase 2 (15 ± 2% vs. 12 ± 1%). Substitution of Na⁺ with Li⁺ resulted in an inhibition of phase 1 (48 ± 4% vs. 32 ± 4%); phase 2, however, was strongly increased (10 ± 2% vs. 18 ± 4%). A similar result was observed when Na⁺ was substituted with NMDG. The ability of PA to relax following re-oxygenation was variably inhibited by Na⁺ substitution, but was not affected by the NCX inhibitors. These results suggest that the phase 1 contraction of HPV involves Ca²⁺ influx via reverse-mode NCX. The involvement of NCX during phase 2 appears to be more complex, but the results are consistent with the possibility that NCX is primarily involved in Ca²⁺ extrusion during phase 2.