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Experimentally-induced hyperthyroidism in rodents is associated with signs and symptoms of pulmonary hypertension. The main objective of the present study was to investigate the effect of thyroxine-induced pulmonary hypertension on the contractile response of the pulmonary artery to 5-HT and the possible underlying signaling pathway. 5-HT concentration-dependently contracted artery segments from control and thyroxine-treated rats with pD2 values of 5.04 ± 0.19 and 5.34 ± 0.14, respectively. The maximum response was significantly greater in artery segments from thyroxine-treated rats. Neither BW 723C86 (5-HT2B-receptor agonist) nor CP 93129 (5-HT1B-receptor agonist) contracted ring segments of the pulmonary artery from control and thyroxine-treated rats at concentrations up to 10− 4 M. There was no significant difference in the level of expression of 5-HT2A-receptor protein between the two groups. Ketanserin (3 × 10− 8 M) produced a rightward shift of the concentration–response curve to 5-HT in both groups with equal potency (− logKB values were 8.1 ± 0.2 and 7.9 ± 0.1 in control and thyroxine-treated rats, respectively). Nifedipine (10− 6 M) inhibited 5-HT-induced contractions in artery segments from control and thyroxine-treated rats and was more effective against 5-HT-induced contraction in artery segments for thyroxine-treated rats. The calcium-activated chloride channel blocker, niflumic acid (10− 4 M) also inhibited 5-HT-induced contractions in artery segments from control and thyroxine-treated rats and was more effective against 5-HT-induced contraction in artery segments for thyroxine-treated rats. It was concluded that hyperthyroidism enhanced 5-HT-induced contractions of the rat pulmonary artery by a mechanism involving increased activity of calcium-activated chloride channels.