A pilot study was undertaken to examine the possible health effects of electromagnetic fields (EMFs). An experimental study was performed for two weeks of preexposure, exposure, postexposure period of February-May 1997 to determine changes in melatonin levels in urine of five volunteers before or after exposure to electric mattress during their normal sleeping hours. An urine sample was collected at about 8 a.m. right after getting up from each volunteer. Average melatonin levels seemed to be 1.6 times lower during EMF exposure period for two weeks than the corresponding levels during preexposure period. Mean melatonin levels in postexposure period were lower than the corresponding levels in preexposure period. It suggested that melatonin levels were likely to take more two weeks for recovering the preexposure melatonin level. Further research is underway with regard to EMF exposure effects on melatonin levels between occupational and nonoccupational groups.