Rat lung microvascular endothelial cells (LMVEC) was cocultured with rat pulmonary arterial smooth muscle cells (PASMC), and the cultures were randomly divided into 4 groups: normal homotypic group (NH), normal coculture group (NC), lipopolysaccharide homotypic group (LH), LPS coculture group (LC). The cell cycles were analysed with flow cytometry. The results showed that after the treatment of 100ng/ml LPS for 16h, the number of cells in G 1 phase was decreased in both homotypic and coculture of LMVEC and PASMC, while that in S+G 2/M were increased. The number of cells in G 1 phase was larger in LC than LH, but the number of cells at S+G 2/M phase was less in LC than LH. Compared with normal group, the unmber of LMVEC at S phase was increased by homotypic and coculture after the treatment with LPS. It is concluded that LPS could effect the cell cycle and accelerate the cell division, and there is a complicated regulatory mechanism in cell proliferation between LMVEC and PASMC.