In the pulmonary artery (PA), hypoxia releases Ca²⁺ from the SR (Dipp et al. 2001) and during hypoxia, Ca²⁺ released from the SR may activate the Ca²⁺-activated Cl^– channel (Cl_Ca) in the plasma membrane (Wang et al. 1997). Decreasing [Cl^–]_o can increase agonist-induced contraction in the PA (Lamb & Barna, 1998). Here we report the effects of decreasing [Cl^–]_o on the hypoxic pulmonary vasoconstriction (HPV) response in preconstricted porcine PA. Porcine lungs were obtained from a local abattoir. Pulmonary arteries (4th-5th order) were dissected free (external diameter 500-800 μm), mounted in a myograph and resting tone set at 2mN. Hepes-based PSS were made hypoxic by aerating with N₂ and P_O2 monitored continuously. In low Cl^– solutions, NaCl was replaced with equimolar Na-gluconate. Pre-tone was achieved using 2 μM PGF_2α. Endothelial function was assessed by determining the ability of 20μM ACh to relax KCl-constricted vessels. All drugs were bath applied. Where appropriate, data are expressed as a % of KCl-induced contraction and presented as mean values ± S.E.M. Significance was determined at P< 0.05 using a Student's paired or unpaired t test as appropriate. Control contractions were induced using 80 mM KCl (14.4 ± 2.8 mN, n=6). PGF_2α-induced contractions were 1.4 ± 1% of control response. Hypoxia (P_O2 < 25 mm Hg) resulted in a slow monophasic increase in tone (13.4 ± 3.6% of control), which was reversible on return to normoxia. In low-Cl^– solution, hypoxia-induced contractions were significantly increased (41.5 ± 9.8% of control; P< 0.05 compared to Cl^–-containing bath solution, n=6). In Ca²⁺-free solution, the hypoxic response (15.9 ± 5.3% of control) was not significantly different compared to hypoxic, normal Cl^– solution, (P> 0.05, n=6). There was no significant difference between endothelial (+) and endothelial (-) vessels in the contractile response to hypoxia, either in normal Cl^– or low-Cl^– solution (P> 0.05). Hypoxia-induced contractions returned to resting tone upon washout with normoxic normal Cl^– solution (1.6 ± 0.3 mN). Washout with normoxic, low-Cl^– solution only resulted in a partial relaxation to baseline (5.1 ± 0.6 mN, P< 0.05, n=5). Increased tone in low-Cl^–, normoxic solution was insensitive to the Ca²⁺ channel blocker nifedipine (5μM) but was sensitive to the Cl^– channel blocker niflumic acid (50 μM). These data suggest a role for Cl_Ca in modulating the porcine HPV response and resting tone in pre-stimulated porcine PA.