It is important to understand the aetiology of interactive mixtures effects (i.e. synergism and antagonism) if results from known cases are to be extrapolated to untested combinations. The key role of toxicokinetics in determining internal concentrations at target sites means that understanding chemical uptake in mixtures is an essential requirement for mechanistic understanding of interactions. In this paper, a combined approach using mixture toxicity testing, toxicokinetic studies and modelling has been used to address the link between joint toxicity and internal concentration. The study is conducted in Lumbricid earthworms with a binary mixture of a metal (nickel) and an organophosphate insecticide (chlorpyrifos) not a priori expected to show interactive toxicity.\ud \ud As expected from their dissimilar modes of action and detoxification, exposure to combinations of nickel and chlorpyrifos resulted in additive toxicity. Measurement of internal concentrations indicated that both chemicals were rapidly accumulated (within 3 days) to equilibrium. When exposed as a mixture, Ni uptake followed the same pattern as found for the single chemical. This was not the case for chlorpyrifos which showed a faster rate of uptake and elimination and a slightly higher equilibrium concentration in a mixture. That the difference in chlorpyrifos kinetics in the mixture did not result in interactive toxicity highlights the need to assess chemical toxicodynamics as well as toxicokinetics. Measurement of chlorpyrifos-oxon identified the presence of this toxic form but implementation of more complex approaches encompassing toxicogenomics and epigenetics are ultimately needed to resolve the toxicokinetic to toxicodynamic link for these chemicals.\ud \u
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