By a variety of exposure routes it is possible that the toxic heavy metals cadmium, arsenic and mercury could enter the diet of farm animals and hence contaminate food products derived from those animals. Therefore, there is a need to be able to assess the likely levels of contamination in animal tissues if exposed to contaminated feed and also to estimate how rapidly an animal will decontaminate once the source of contamination is removed from the feed. The development of dynamic models to predict changes in the degree of heavy metal contamination in tissues of ruminants have been hindered by the lack of data on the transfer and excretion rates of these metals from tissues. A study is described during which dairy cows were given a single intraruminal administration of 109Cd, 73As and 203Hg and measurements made of the subsequent concentrations of the radioisotopes in body tissues and milk. The resultant data were used to adapt previously developed compartment models describing the behaviour of the metals in sheep for use with dairy cows. Two changes were made to the sheep models: (i) a new excretion route was included to describe transfer to milk; (ii) the rate coefficients (with the exception of those involving gut absorption and transfer) were adjusted according to the ratio of the metabolic live-weights (live-weight (kg)0·75) of the sheep to that of the cattle. The models predicted levels of the metals in the cattle tissues and milk reasonably well.