A continuous recording of carbon dioxide emission (VCO2) was used to quantify the respiratory responses of grouped male GFF mice, LD12:12 synchronized and submitted to one hour of acoustic stimulation. It was shown that, both in percentage and amplitude of VCO2variations, the highest and most frequent responses were: (1) during the dark (D) when mice are more active, (2) at the beginning of stimulation vs. at the end. Moreover, ultradian respiratory variations of the groups influence the number of the responses to the acoustical stimuli.