The complex binding of cadmium ions to humic and fulvic acids in water may influence the absorption and distribution of drinking-water Cd in humans. Thus, in the present study mice were given a single oral dose of Cd (109CdCl2, 25 microg/l) in 100 microl Millipore water containing different concentrations of humic compounds (0, 1, 10 and 100 mg dissolved organic carbon/l). The complex binding of Cd was studied by dialysis. At neutral pH, 1 mg dissolved organic carbon/l caused complex binding of more than 50% of the Cd, whereas more than 90% of Cd was bound at 10 and 100 mg dissolved organic carbon/l. At pH 3 the complex binding of Cd decreased somewhat, but over 90% of the Cd was bound at 100 mg dissolved organic carbon/l. Complex binding of Cd increased the lipid solubility of Cd, expressed as an octanol/ water partition coefficient, Nevertheless, more than 99% of the bound Cd was present as hydrophilic binding forms. Irrespective of the bound of Cd, the intestinal uptake and intracellular distribution (gel filtration on Sephadex G-75 column) were not affected by the humic substances 6 hr after dosage. Moreover, complex binding did not influence the intestinal absorption of Cd 24 hr after exposure. The median Cd retention in the kidneys of the 100 mg dissolved organic carbon/l group was 23% and 46% lower than that of the control group 6 and 24 hr after administration, respectively, indicating alterations in the distribution of Cd after absorption. Thus humic substances may affect the metabolism of toxic heavy metals, such as Cd, in vivo in mice, indicating that the presence of humic and fulvic acids in drinking water should be considered in future risk assessments of metals in drinking water.