Abstract: In this paper the attenuation characteristics of 8 recorded premonitory geo-sound signals of earthquakes in sand layer is measured, and the effect on the metabolic process of 5-hydroxytryptamine (5-HT) in the brains of mice is analysyed. The results are as follow:1.The forms of the attenuation characteristics of 8 geo-sound signals in sand layer are basically similar, showing that geo-sound signals have almost the same fundamental characteristics. The attenuation is obviously related to frequency within the frequency bands of 50-100 Hz and 250-630 Hz, but attenuation is relatively uniform in the dominant band of 100-250 Hz, that is, the absorption coefficient of a sand layer with a thickness of 20-30 cm is about 410-2- 610-2 dB/cm within this band.2.The relative attenuate rate of intensity level of geo-sound signals transmitted into a hole through a sand layer 30 cm thick shows some decrease as compared with attenuatim in sand of the same depth, and it is related to the fundamental characteristics of the geo-sound signal. Within the dominant band, single tumbling sound, continual sound of roasting beans and so on, and blast sound drop 4.40.8=, 3.50.3% and 3.00.2% respectively, but the drop in intensity of coninuous sound of snare drums is not obvious.3. Two groups of mice quiet adaptation of 48 hours are exposed for 30 minutes to random geo-sound signal noise of 105-85 dB and of 76-66 dB respectively. It was found that the accumulative volume of 5-hydroxyindole-3-acetie acid (5-HIAA), that is, the metabolite of 5-HT in the centre of both groups increases respectively 24.7% and 262.2% as compared with animals of the contsrol group.These results show that the spectrum characteristics of geo-sound with premonitory property transmitted into the ground is related to the absorption characteristics of the ground medium, and it is the stimulation by low frequency noise that makes the animals feel very uneasy. Though the dominant band of geo-sound transmitted into hole is obviously lower than the sensitivie band of hearing of mice. It may directly effect the metabolic process of 5-HT in the brains of mice. Moreover, acoustic effect of the hole may raise the ability of animals to distinguish sound signals above the noise background. It may be seen that geo-sound with premonitory property may be an important factor which causes the abnormal behavior of animals living in holes, eg. mice, prior to the occurrence of earthquakes.