To clarify the effects of infrasound on gastric mucosal blood flow, rats were exposed to infrasound for 20 minutes. The sounds were pure tones of 8, 16 and 32 Hz, at sound levels ranging from 80 dB to 130 dB. Gastric mucosal blood flow was determined by the inhaled hydrogen gas clearance method. Norepinephrine and epinephrine concentrations in the plasma were also measured. The exposed rats showed decreased gastric mucosal blood flow with increasing sound levels of infrasound at each octave-band frequency. To significantly decrease the gastric mucosal blood flow, infrasound at 32, 16 and 8 Hz needed sound levels of 100, 110 and 130 dB, respectively. These findings suggest that, as the frequency of infrasound decreases, an increased sound level is necessary to decrease the gastric mucosal blood flow. The norepinephrine and epinephrine concentrations in the plasma were 0.65±0.47n.g/ml and 0.35±0.43 ng/ml, respectively, in the control rats, while the corresponding values were 0.91±0.87 ng/ml and 0.74±0.81 ng/ml, respectively, in the exposed rats. There were significant increases (p>0.05) in norepinephrine and epinephrine. Thus, it is probable that exposure to infrasound stimulates the sympathetic nervous system and causes a decrease in gastric mucosal blood flow.