At Tampere University of Technology, the ELF electric and magnetic fields have been studied for several years. The studies have included measurements and calculations, as well as exposure assessments. In previous studies at TUT, electric and magnetic fields at 400 kV and 110 kV air insulated substations were measured. The aim of this study was to measure and analyze the electric and magnetic fields at a gas insulated substations (GIS) using long-term and instantaneous measurements. In addition to field strength measurements, also the harmonic contents of the fields were examined. The highest instantaneous magnetic field value (173 /spl mu/T, RMS) was measured in the cable subway behind the switchyard. This value is below the ICNIRP (International Commission on Non-Ionizing Radiation Protection) occupational guideline (500 /spl mu/T at 50 Hz). The highest electric field value was only 0.002 kV/m, which is well below the ICNIRP occupational guideline (10 kV/m at 50 Hz). In these measurements the harmonic content was found to be insignificant.