AIM: The effect of urotensin II (U-II) on proliferation of cultured pulmonary arterial smooth muscle cells (PASMCs) of rabbits and its mechanism are investigated. METHODS: PASMCs were isolated using explant technique. RPASMCs were incubated in serum-free medium with different concentrations of nicardipine, calcimodulin antagonist W 7, PKC inhibitor H 7 or MAPK inhibitor (PD 98059 ), with or without U-II. RPASMC proliferation was examined by MTT [3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay and by the increase in [ 3H]-thymidine incorporation into DNA. RESULTS: U-II (10 -9 mol/L-10 -7 mol/L) increased A value of PASMCs by MTT assay and [ 3H]-thymidine incorporation in PASMCs in a dose-dependent manner. U-II induced a maximal effect at a concentration of 10 -7 mol/L. A value and [ 3H]-thymidine incorporation rose 42 9% and 68 5% ( P