At the end of one-month-long period of very low outside temperatures (January 2017), air temperature was measured at 74 sites in 25 km long Postojna-Planina Cave System. Spatial data were further interpolated in GIS Surfer (Golden Software) to 10×10 m grid. Results show strong impact of ventilation that reduces average temperature of the karst massif (8.7-11.4 °C) to nearly freezing temperature close to the entrance. Additionally, strong impact is recognized by allogenic underground river that reduces air temperature of the middle part of cave system to about 4 °C. On the other hand, weakly ventilated passages indicate very stagnat temperature with les than 0.1 °C of annual variation. This study shows very varied conditions for underground fauna despite general belief that annual air temperature in caves is very constant. Measurements will be repeated at the end of summer when the highest temperatures are expected to calculate maximal annual temperature variation.