A seasonal heat storage system was constructed in 1982-83 at Lulea University of Technology. The heat extraction from the storage system, 2 GWh, corresponds to the heat demand of 100 one-family-houses in northern Sweden. The store has been in operation since July, 1983. The borehole heat store consists of a rock volume of 100,000 m**3, perforated by 120 vertical boreholes to a depth of 65m. The heat is stored in the rock volume itself. The boreholes work as heat exchangers. During the summer the heat store is supplied with heat via the district heating network. During the winter the heat is utilized for heating of one of the university buildings. Construction, operation and functioning of the heat store is evaluated in a research project conducted by WREL.