The change in the heart rate was known to show the activity of autonomic nervous system in the human heart. In the present study, we conducted to analyze the fish heart rate change and fluctuation of interbeat under stimulated conditions. The rainbow trout were exposed to a continuous light (20, 000lx), sound (130dB, 300Hz) and repetition of these stimuli. Mean heart rate was calculated, and also the computed power spectrum density of interbeat valiability. Continuous light stimulus caused an increase in the heart rate. But the heart rate was not affected by continuous sound stimulus. It was nearly the same as the resting one. Nevertheless, alternate light and sound stimuli caused an increase in the heart rate. The R-R intervals were variable when the fish were resting, but remain stable when exposed to the continuous light or alternate light and sound stimuli. In the condition exposed to those light and sound stressor, the power spectrum of R-R intervals were dominant in relatively low frequency components. It was supposed that the tachycardia and stable interbeat valiability were sympathetic nerve contribution.
