Aim. The aim of this study was to measure aerobic demands of fire fighting activities including exercise in the heat. Methods. Twenty-two experienced firefighters performed the Trondheim test simulating fire fighting tasks including work in the heat. Maximal oxygen uptake (VO2 max), heart rate (HR) and ventilation were recorded continuously. Data were compared with results obtained during a treadmill test during which the participants were dressed as smoke divers. Results. The participants completed physical parts of the Trondheim test in ~12 min (range: 7.5–17.4). Time to complete the test was closely related to the participant’s VO2max. HR of ~170 beats/min and pulmonary ventilation of ~100 L/min were higher than at lactate threshold (LT) during laboratory tests. VO2 averaged over the test’s physical part was 35 ± 7 ml/min/kg, which was at the same or below the level corresponding to the participants’ LT. Physically fit participants completed the test faster than less fit participants. Slower and physically less fit participants consumed more air and used more oxygen than faster and physically more fit participants. Conclusion. The Trondheim test is physically demanding; it distinguishes physically fit and less fit participants.