Sidescan sonar and TV/photo surveys, TV-controlled sulphide grab sampling and sediment sampling have been carried out at the TAG and MARK hydrothermal fields of the Mid-Atlantic Ridge during research cruises of the Sevmorgeologija Association, St Petersburg. Detailed mapping of the MARK field (23°N) allowed contouring of the massive sulphide bodies. The largest of them are Cu-enriched and the smallest zincenriched. Sulphides of the TAG field (26°N) active mound appear to be Cu-rich in its central and Zn-rich in its marginal parts. Sampling of the inactive Mir hydrothermal mound of the TAG field revealed an enrichment in Cu in its northern part and an abundance of silica in the south. This mound and especially the north hydrothermal zone, both presently inactive, determined the metal distribution in TAG sediments during the last 10 000-13 000 years. Extensive Cu-rich sulphide deposits were also discovered and sampled near 14°45'N. The dominance of Cu over Zn in mature Mid-Atlantic Ridge hydrothermal deposits, arising at late stages of their formation, can probably be explained by dominant leaching of Cu from basalts at advanced stages of development of the hydrothermal systems and/or by increasing the efficiency of Cu sulphide deposition in the course of formation of the ore body.