Background: The aim of the study was to evaluate serum levels of the target enzyme for H₂S toxicity – cytochrome c oxidase (COX) and enzymes involved in the synthesis of H₂S – cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) in copper mine miners. Material and Methods: The initial and basic study was conducted respectively in 237 and 88 miners, working in 2 mining shafts: I – no H₂S emissions recorded in the last 10 years (study group A) and II – H₂S emissions occurred (study group B). A medical examination was performed and 10 ml of blood was collected from miners immediately after exiting the mine. Results: There were no clinical or biochemical changes typical for H₂S toxicity. Sulfhemoglobine was undetectable and there were no changes in the red-ox system. However, in group B, regulatory changes were found; a tendency to higher concentration of CBS and CSE, a higher activity of angiotensin converting enzyme (ACE) compared to group A (p < 0.05) and a linear relationship between ACE and CSE (r = 0.6927; p < 0.001). It has been shown that cigarette smoking decreases COX (p < 0.05), however, in miners working in shaft II, the decreased level of COX may result also from the presence of H₂S in the gaseous emissions. Conclusions: COX concentration can be a sensitive indicator of exposure to H₂S. The measurements of blood H₂S concentrations carried out in workplaces should explain the cause of the changes observed in the COX, CBS and CSE activity. Med Pr 2015;66(4):539–548