Concern about the health effects of extremely low frequency (ELF) magnetic fields (MF) has been raised by epidemiological studies indicating an association between certain cancers and living near power lines or working in high electric field environments. Alterations in pineal function have been proposed as a mechanism through which power-frequency MFs may interact with living organisms. A double blind laboratory study was performed to evaluate daytime exposure effects of 100 microT root mean square (rms) 50 Hz MF. Three head exposure sessions of 30 min each were performed: sham, continuous, and intermittent (15 s on/off cycles) MFs were presented to each subject in early or late afternoon (13:30 or 16:30 hr). Twenty-one healthy male volunteers (20-27 yr old) participated in these 3-weekly experimental conditions. Blood samples were drawn for serum melatonin measurement, hourly at night (from 20:00 to 07:00 hr) under controlled environmental conditions. Urinary excretion of 6-sulfatoxymelatonin (aMT6s), the main melatonin metabolite, was measured for a 17 hr period, by means of urine samples taken at 19:00 hr (14:00-19:00 hr "afternoon period"), 23:00 hr (19:00-23:00 hr "evening period"), and 07:00 hr, day 2 (23:00-07:00 hr day 2 "night-time period"). There were no significant differences in either plasma melatonin or in aMT6s excretion profiles in the three experimental conditions. However, a tendency for a smaller increase of night-time urinary aMT6s after continuous MF exposure was found (P=0.08) particularly in men with the lower excretion rate of aMT6s ("Low Group") (P=0.07). We conclude that this study does not indicate that daytime acute MF exposure influences either melatonin secretion or aMT6s excretion. Inter-individual differences in pineal production of melatonin, however, have to be taken into account in further studies.