Hypoxia induces a transient contractile response of the porcine pulmonary vein (PV)(1). In 3rd-4th order intrapulmonary veins, this contraction is followed by a relaxation to below baseline while subsequent exposure to hyperoxia produces a second contractile response (unpublished observations). The underlying cellular effectors of this contraction in response to high oxygen conditions remain unclear, though hyperoxia has been shown to result in medial thickening and lumen occlusion of rat PVs(2). Porcine lungs were obtained from a local abattoir. Size-matched 3rd order superior and 4th order inferior pulmonary veins (mean external diameter 3.3± 0.4 mm, n=3-4) were dissected free and mounted in a four channel myobath. Resting tone was 1.5g. KREB’s-based bath solutions were gassed with 95% O2/5% CO2 (hyperoxia) or 95% N2/5% CO2 (hypoxia). Hypoxic and hyperoxic contractions were induced in the absence of pharmacological pre-tone. PVs samples were snap frozen in liquid N2 and K+ channel expression identified using end point RT-PCR. Data are expressed as a % of KCl-induced contraction and presented as mean values ± S.E.M. Statistical significance was determined at P< 0.05 using a Students unpaired t-test. Under control conditions, hyperoxia did not induce a contractile response in either superior or inferior PV rings. Hypoxia induced a transient, monophasic contractile response in 89% of PV rings. There was no significant difference in the hypoxia-induced venoconstriction between superior and inferior PVs (37±12 (n=4) and 35±15%KCl (n=3), respectively; P>0.05). and 76% of the veins then relaxed to below baseline tension. All PV rings tested contracted when gassing was reverted from hypoxia to hyperoxia. There was no significant difference in the maximal contractile response between superior and inferior PVs (37±16%KCl for superior (n=4) and 24±7%KCl for inferior PVs (n=3);P>0.05). The large conductance Ca2+-activated K+ channel (BKCa) blocker Penitrem A(1) (500nM) inhibited hypoxia-induced relaxation and pre-incubation with 2 μM ryanodine (known to inhibit SR Ca2+ release(3)) prevented both the hypoxia-induced relaxation and hyperoxic-induced contraction. RT-PCR and product sequencing identified a truncated BKCa Zero and STREX splice variant in PVs (n=3)(1). Hyperoxic-induced contraction and hypoxia-induced relaxation in 3rd-4th order PVs require an intact ryanodine-sensitive Ca2+ source. The underlying mechanisms and regulatory pathways require clarification but may involve the activation of BKCa channels.