In plants the essential biochemical role of molybdenum in the processes of nitrogen fixation and nitrate reduction is well recognized but in animals the biochemical role and importance of molybdenum are less certain and reports of the metabolic effects of molybdenum are sometimes con flicting. Processes which are certainly influenced by molybdenum are the oxidation of purines, synthesis of proteins, hydrolysis of phosphate esters, oxidation of sulfide and other aspects of sulfur metabolism, and the transport and utilisation of iron. The metabolic effect of molybdenum at low molybdenum concentrations may not be apparent at higher concentrations or may even be reversed (e.g., protein synthesis and animal growth). It is also important to realise that the metabolic effects of molybdenum cannot properly be understood without taking account of the metabolic relationships of molybdenum and other trace elements (e.g., molybdenum-copper antagonism).