To study the changes of pulmonary surfactant associated protein A (SPA) of lung tissue in acute pulmonary embolism (PE).Male SD rats were injected with medical gelfoam microspheres via jugular vein to induce the PE model. Thirty-two rats were randomized into four groups: a control group (n = 8) and three groups of embolism for 24 h (n = 8), 1 week (n = 8) and 2 weeks (n = 8), respectively. The rats were sacrificed at the time of 2 weeks, 24 h, 1 week and 2 weeks. Pulmonary artery pressure, heart rate and respiratory rate were measured by right heart catheterization and artery blood gas was analyzed at the time of sacrifice. Lung tissues were sliced and stained with HE to observe the embolism of pulmonary arteries. RT-PCR and Western blot were used to study the changes of SPA mRNA and SPA protein in lung tissues.In the embolism rats, the pulmonary arterial pressure (mm Hg) increased significantly; the mean pulmonary arterial pressures of the 4 groups were 14.2 +/- 4.1, 26.1 +/- 7.5, 26.1 +/- 6.8 and 29.0 +/- 8.2, respectively (P < 0.05). The heart rate (per/minute) also increased significantly, 415 +/- 15, 451 +/- 35, 463 +/- 29 and 446 +/- 14, respectively in the 4 groups (P < 0.05). The arterial partial pressure of oxygen (mm Hg) decreased significantly, 94.1 +/- 8.8, 80.5 +/- 5.8, 80.4 +/- 13.8 and 73.4 +/- 14.3, respectively in the 4 groups (P < 0.05). After 24 h of embolism, pulmonary arteries were shown to be embolized with gelfoam, and later the gelfoam was resolved at week 2. The expression of SPA mRNA and protein in lung tissues decreased significantly after embolism; the mRNA level was 1.43 +/- 0.51, 0.83 +/- 0.33, 0.91 +/- 0.33 and 0.87 +/- 0.35 respectively in the 4 groups (P < 0.05); the protein level was, 1.00 +/- 0.00, 0.44 +/- 0.18, 0.44 +/- 0.33, and 0.52 +/- 0.32, respectively (P < 0.05).The SPA level decreases significantly in acute pulmonary embolism, which may play an important role in hypoxemia in pulmonary embolism.