In an attempt to define a tolerable daily intake (TDI) for molybdenum based on a toxicological risk analysis approach, a large literature survey was conducted. In man, absorption of molybdenum after oral intake is in the range of 28-77% and urinary excretion is 17-80% of the total dose. A low order of toxicity of molybdenum compounds has been observed in humans. However, with the available data, it is not possible to calculate any dose-response or dose-effect relationships. Because molybdenum toxicity is associated with copper intake or depleted copper stores in the body, humans who have an inadequate intake of dietary copper or some dysfunction in their copper metabolism that makes them copper-deficient could be at greater risk of molybdenum toxicity. In the absence of relevant human studies, animal studies were evaluated for the derivation of the TDI. Effects of Mo on reproduction and foetal development were found to be critical effects observed in rats and mice. A dose-response relationship was observed in a study by Fungwe et al., with a 'no observed adverse effect' level (NOAEL) and a 'lowest observed adverse effect' level (LOAEL) of 0.9 and 1.6 mg Mo kg(-1) day(-1), respectively. Applying uncertainty factors of 10 for intraspecies and 10 for interspecies differences to the NOAEL, a TDI of 0.009 mg Mo kg(-1) day(-1) was calculated. The TDI is given a medium confidence rating. This TDI is more than double the upper limit of adequate intake for adolescents and adults that was derived from the Mo content of the average diet in the USA.