The aim of this study was an analysis of the relationship between pineal hormone melatonin level and long-term occupational exposure to elemental mercury vapour (Hg°) of ex-mercury miners. Melatonin (MEL) is a hormonal product of the pineal gland. Hg accumulation in the pineal gland in ex-miners could modify the synthesis of melatonin and there are no data available in the scientific literature on the possible effects of Hg on melatonin excretion. Regression/model trees were used to perform the analyses and investigate the interactions between melatonin secretion and excretion on one hand, and various indicators of occupational Hg exposure and some other medical variables on the other. The results point us towards some previously unknown factors that influence the excretion of melatonin and support some assumptions that melatonin also has antioxidative effect. In the process of antioxidation activity melatonin decomposes and we assume this is the reason for decreased excretion of melatonin in mercury miners.