Development of the deep deposits of minerals is inevitably associated with an increase in the temperature of the rock mass and, as a result, an increase in the air temperature in the workings. Working in such conditions reduces the productivity of labour and leads to a deterioration in health of the underground personnel. In the world practice, complex indicators of heat stress are used to accurately assess the microclimate. In the Russian Federation, the index of environmental heat load is most widely used. The article is devoted to the study of microclimatic working conditions of the underground workers of a deep polymetallic mine located in the northern part of the country. As part of the study, all the professions involved in the excavation of a preparatory dead-end development were considered. Among them: a mine foreman, load-haul-dump machine driver, stope miner, a sinker, a driller. At the same time, the microclimatic parameters were continuously monitored at the locations of employees, the heart rate of each of them was constantly recorded, and the type of work performed was also recorded. According to the research results, four out of five professions are classified as a harmful class of working conditions. The most unfavorable conditions were recorded at the sinker. This is due to the long stay of the employee in the dead end during the shift and the severity of the operations performed. The most favorable conditions were recorded for the driver of a load haul dumper, the cabin of which is equipped with an air conditioner.