Objective. To investigate the characteristics of urotensin II (U-II) receptor in the rat airway smooth muscleand the effect and signal transduction pathway of U-II on the proliferation of airway smooth muscle cells.Methods. Using 125-UII binding assay to measure the Bmax and Kd of U-II receptor. Using the 3H-TdRincorporation to deter mine the effect of U-II on the proliferation of airway smooth muscle cells and its signal transduc-tion pathway. Using Fura-2/AM to measure the effect of U-II on the cytosolic free calcium concentration.Results. 1. 125I-UⅡ binding increased with the time and reached saturation at 45min. The Bmax was(ll. 36 +0.37)fmol/mg pr and Kd was (4.46 +0.61)nmol/L. 2. U-II increased 3H-TdR incorporation of theairway smooth muscle cells in a dose-dependent manner. 3. H7, PDg8059 and nicardipine, inhibitors of PKC,MAPK, calcium cha.nnel, respectively, significantly inhibited U-II-stimulated 3H-TdR incorporation of airwaysmooth muscle cells. W7, inhibitor of CaM-PK, had no effect. 4. Cyclosporin A, inhibitor of CaN, inhibited3H-TdRincorporation ofthe airway smooth muscle cells induced by U-Ⅱl in a dose-dependent manner. 5. U-Ⅱlpromot-ed cy-tosolic free calcium concentration increase by 18%.Conclusions. 1. There was U-II receptor in the rat airway smooth muscle. 2. The effect of U-II-stimulated-3H-TdR incorporation of airway smooth muscle cells was mediated by such signal transduction pathway as Ca2 +.PKC, MAPK and Ca.N, etc.``