The study was performed in a sample of 39 firemen. The state of health for each fire fighter was evaluated by means of a clinical examination and also through some specific exams. Aerobic capacity was estimated as an indicator of physical fitness. In each load, cardiac frequency and oxygen consumption were measured under a steady state condition. Additionally, body composition was calculated using a Tanita professional scale. Physical effort at work was determined by measuring cardiac frequency using a telemetric unit. Evaluations were carried out during night shifts, registering information under the following conditions: night shifts without emergency, fire simulation, training exercises and real fire fighting. In general terms, it is possible to state that fire fighters are healthy according to the result of the clinical examination. However, it is very important to highlight that 70% of the firemen consume alcohol and 80% were smokers. The average aerobic capacity of the sample was 2.6 l/min or 34.5 ml/kg/min. In terms of overweight and obesity, the body mass index was 25.7, whereas the percentage fat mass reached an average of 22.9 %. The cardiovascular load in the night shifts without emergencies did not surpass 40%, which in Chile is considered the highest limit for sustained work. However, during training, simulations and real fires, the firemen had higher cardiac frequencies, reaching peak levels close to 100% cardiovascular load.