AIM: To evaluate the prophylactic effect of aminophylline on the pulmonary hypertension rebound reaction to exposure to NO in the hypoxic pigs. METHODS: The 10 pigs undergone Swan-Ganz catheter, the mPAP was measured with a Physio-recording instrument and PaO 2 was measured with a blood gas analyzer, when breathing NO for 30 minutes and suddenly stopping breathing NO, administing aminophylline 0.25 g, followed by 30 minutes with room air. The respiratory rate and heart rate were also monitorried with a Hewlett-Packard portable monitor. RESULTS: The mPAP of the acute hypoxic pig was induced significantly after breathing 10 -5 NO. When suddenly stopping breathing NO, the induced mPAP became more and more high, the level of the mPAP in 5 minutes was similar to the values before absorbing NO, the mPAP in 10 minutes was higher than values before absorbing NO, while the level of PaO 2 was lower than the values before absorbing NO; but suddenly stopping breathing NO, administing aminophylline, although the induced mPAP became high, the speed was slower than the controls, the level of the mPAP in 30 minutes still was lower than the values before absorbing NO. CONCLUSION: Aminophylline has preventive effects on the pulmonary hypertension rebound reaction to exposure to NO in the hypoxic pigs.