This research was designed to analyze the effects of heroin overdose on pulmonary perfusion in dogs. In view of this pulmonary perfusion by radionuclide method, arterial blood pressure by the femoral artery, mean right ventricular pressure and pulmonary wedge pressure by Swang-Ganz catheter were detected in 7 medium sized healthy dogs. In addition PaO2, SaO2, PaCO2 and blood pH were also determined by current methods. All the dogs received an acute bolus of purified heroin at dosage of 12.5 mg/Kg. All the measurements were made at baseline and at 30 seconds, 10 minutes, 20 minutes and 30 minutes after heroin administration. Our data indicated an impaired pulmonary perfusion by the analysis of the slope and the area under the time/activity curve following heroin overdose. In addition a significant reduction (p < 0.001) in blood pressure and a significant rise (p < 0.01) in mean right ventricular pressure and in pulmonary wedge pressure were observed after 30 seconds by heroin overdose. Moreover heroin administration promoted a significant increase (p < 0.001) in PaCO2 and a significant decrease (p < 0.001) in both PaO2 and SaO2. These results indicated that early pulmonary perfusion abnormalities may be recognized in dogs receiving an acute heroin overdose and they were associated with impaired gas-exchange. These changes in respiratory function could represent and important mechanism of clinical manifestations following heroin overdose.