Currently there is a need for an accurate and non-hazardous method to measure individual intake of a supplement in grazing sheep over a prolonged period. This paper examines the potential of fenbendazole (FBZ) as a marker of intake. The following five experiments aim to determine the relationship between oral ingestion of FBZ and the plasma concentrations of FBZ and its metabolites oxfendazole (OFZ) and FBZ-sulfone (SUL) after single, multiple and daily doses both in housed and grazing sheep and sheep infected with internal parasites. The results from these experiments indicate that OFZ+SUL concentrations in plasma are dependent on FBZ dose rate in housed and grazing animals with differences evident between different dose rates (P < 0.001). Variability of OFZ and SUL concentrations increase in grazing compared with housed animals. Area under the curve of metabolite concentrations was also shown to indicate dose rate regardless of the timing and frequency of dose. Stepwise regressions indicated that sampling every 48 h gave a good representation of area under the curve for different dose rates (R² = 0.951, P < 0.001). A significant separation of treatment means was achieved when samples were taken every 48 h and pooled during daily dosing with FBZ (P < 0.001). Finally gastrointestinal nematode infection did not affect OFZ and SUL concentrations after daily doses of FBZ. The results from these experiments indicate that FBZ is a useful and accurate marker of supplement intake in grazing animals.