Three parasites(Moniezia expansa, Haemonchus contortus, andFasciola hepatica), removed from mebendazole-treated sheep at arbitrarily chosen times after treatment, showed detrimental changes in energy metabolism. The changes produced inM. expansaincluded a reduction in total nucleotides, ATP levels and ATP/ADP ratios, that matched observations madein vitro. Scoleces were found to be more acutely stressed than gravid segments and there was no destrobilation. Similar changes were recorded inH. contortus, though it was evident that neither of the times chosen for assays coincided with maximum drug effect. InF. hepaticathe total nucleotide concentration and ATP/ADP ratio increased up to 18 hours after treatment, but after 30 hours both values were significantly depressed.