A suppression in melatonin secretion is one of the mechanisms proposed to explain the possible adverse effects of extremely low frequency magnetic fields (ELF-MF), but the results of research are inconclusive. This study investigated the effect of occupational ELF-MF exposure on 6-sulfatoximelatonin (6-OHMS). Exposure was monitored for three complete work shifts in 59 workers using personal exposure meters. Environmental exposure was also evaluated. Urinary 6-OHMS in morning samples, an indicator of night-time melatonin production, was measured. Urine was collected twice on Friday and the following Monday. Workers were classified according to ELF exposure as low exposed (≤0.2 µT) or higher exposed (>0.2 µT): 6-OHMS did not differ between groups (P > .05) in either Friday or Monday urine samples. In addition, 6-OHMS was not related to exposure under multivariate analysis. The ratio between 6-OHMS in Monday versus Friday samples was also calculated to test the hypothesis of a possible variation in pineal function after 2 days, interruption of occupational ELF-MF exposure: again no exposure-related difference was observed. Our results do not support the hypothesis that occupational exposure to ELF-MF significantly influences melatonin secretion. Bioelectromagnetics 27:667–673, 2006. © 2006 Wiley-Liss, Inc.