A multi-gas photoacoustic sensor was developed In order to monitor microclimatic parameters at ppm level from the atmosphere of Individually ventilated cages housing mice. Ammonia, water vapour and carbon dioxide were measured on-line and in real-time during two weeks and 24 hours a day. The quality of bedding and the ventilation rate inside the cages has been monitored. The circadian activity level of these animals has then been observed.