Objective To investigate the influence of pulmonary microembolism of early stage on hemodynamics, respiratory function and blood coagulation in dogs. Methods Eight mongrel dogs (6 male, 2 female) weighing 15.5-16.5 kg were anesthetized with intravenous atropine 0.02 mg?kg-1 , propofol 2-3 mg?kg-1 and pancuronium 0.4 mg?kg-1 . The animals were intubated and mechanically ventilated with 100% O2. The ventilatory settings were as follows : VT 12 ml? kg-1 , RR 15 bpm and I: E = 1:2. Anesthesia was maintained with intravenous infusion of propofol at 200-300 ?g ? kg-1? min-1 and intermittent iv boluses of pancuronium. A 7.5 F Swan-Ganz catheter was placed via femoral vein for hemodynamic monitoring and injection of microemboli. 50 ml of blood was removed from artery and mixed with methylene blue. The clot was cut into small pieces 1-2 mm in diameter. After being washed with normal saline, 100 microemboli in normal saline 10 ml were rapidly injected into pulmonary artery via Swan-Ganz catheter. BP, HR, pulmonary arterial pressure (PAP), pulmonary arterial wedge pressure(PAWP), arterial blood gases, airway pressure, lung compliance, D-dimer, tissue plasminogen activator (t-PA), protein C and S were measured and recorded before (T0 ) , immediately after (T, ) and 30 min (T2) , 60 min (T3 ) , 120 min (T4) after embolization. Two hours after embolization, chest was opened and lung tissue was obtained for microscopic examination. Results Both systolic and diastolic PAP, PAWP and pulmonary vascular resistance (PVR) were significantly increased immediately after embolization (T1 ), and then decreased to the baseline level (T0) at Ih after embolization (T3) . There were no significant changes in respiratory function. D-dimer was increased at 30 min after embolizatiion (T2 ) and decreased to the baseline level at T4 . Microscopic examination showed that the lung exhibited hemorrhage and consolidation with microemboli in arterioles. Conclusion Pulmonary microembolism induces pulmonary hypertension and change in D-dimer level in the early stage but respiratory function is not affected. It causes injury to the lung parenchyma.