When endogenous nucleotides are released within the vascular lumen they can cause vasodilation via P2Y receptors on endothelial cells, but if the endothelium is damaged then they may evoke vasoconstriction via P2Y receptors on smooth muscle cells (Burnstock & Knight, 2004). The P2Y agonists ATP, UTP and UDP evoke vasoconstriction in rat pulmonary artery(Chootip et al., 2002), but little is known about the signalling mechanisms through which they act. The aim of this study was to establish the roles of Ca²⁺ signalling pathways in these P2Y receptor-evoked contractions. Small intrapulmonary arteries were dissected post mortem from rats (200-300g). The endothelium was removed and the arteries mounted on intraluminal wires under isometric conditions. UTP and UDP (both 300 μM) evoked slowly developing, tonic contractions, which reached a peak within 5 minutes. 300 μM ATP also evoked a slowly developing contraction, but this response was not maintained and decayed back to close to basal levels over the same time-course. Upon removal of extracellular Ca²⁺, the contractile response of each nucleotide was significantly decreased (P < 0.001) by 50-80 %. The responses of UTP and UDP in the Ca²⁺-free salt solution were significantly larger than the response of ATP in the same conditions (P < 0.01). In the presence of the Cav1.2 channel blocker, nifedipine (1 μM), the responses to ATP, UTP and UDP were significantly (P < 0.001) decreased by around 50%. The role of Cav1.2 channels on maintaining nucleotide-evoked contractions was also investigated. Nifedipine was added when the contraction to UTP or UDP had decreased from the peak and reached a plateau. Nifedipine caused a significant relaxation of the maintained contraction of both UTP and UDP such that they were 20-30% of the control response over the same time course, but in the absence of nifedipine (P < 0.001). Pre-administration of niflumic acid (10 μM), a blocker of Ca²⁺-dependent Cl^– channels, significantly depressed the responses to ATP, UTP and UDP to approximately 40% of control ( P < 0.001). Addition of niflumic acid (10 μM), 20 min after UTP and UDP had been administered and their contractions had reached a plateau, caused a significant inhibition in both cases (P < 0.001). There was a more marked effect on UTP compared to UDP, but this difference was not significant. These results indicate that influx of extracellular Ca²⁺ via Cav1.2 channels, release of Ca²⁺ from intracellular stores and Ca²⁺-dependent Cl^– channels play a significant role in the induction of the P2Y-mediated pulmonary vasoconstriction. Furthermore, these data provides evidence that Cav1.2 channels and Ca²⁺-dependent Cl^– channels are important in the maintenance of pulmonary artery contraction in response to UTP and UDP.