Netobimin and albendazole were administered to 3-month-old lambs with moderate infections of Nematodirus battus and to comparable parasite-naive lambs. Albendazole sulphoxide and albendazole sulphone concentrations were determined in the plasma of all lambs at frequent intervals after treatment. Both anthelmintic preparations were 100% effective in reducing nematode faecal egg output in the lambs. There were no significant differences in the concentrations of the sulphoxide or sulphone metabolites in parasitized compared with non-parasitized lambs given the same parent anthelmintic. The parasite-naive lambs were subsequently weaned and maintained indoors in conditions designed to preclude nematode parasite infection until they were 9 months old. Netobimin and albendazole were administered again and the plasma profiles of the albendazole sulphoxide and albendazole sulphone metabolites determined. There were no significant differences in the plasma distribution of these metabolites with age of the lambs. The area under the plasma concentration time curve, mean resident time and apparent half-life of the albendazole sulphoxide metabolite was determined following administration of each parent drug and the clearance of the metabolite/systemic availability of parent drug was determined as a marker of the amount of drug available for metabolism. There were no significant differences in pharmacokinetic variables between parasitized and non-parasitized animals nor with the age of the animals.