In this paper are presented the measurements of radon concentration ta
ken in the period 1991 - May 1994 in the Gran Sasso Underground Labora
tory, Assergi (L'Aquila), Italy. The radon level was reduced to accept
able values from the point of view of radiation protection and low bac
kground radiation to the experiments. The radon concentration depends
strongly on the ventilation of the laboratory.