The effects of ivermectin and ivermectin aglycone on pharyngeal uptake of a carbohydrate substrate (3H-inulin) were measured in larvae of a macrocyclic lactone (ML)-susceptible isolate and 2 ML-resistant isolates of Haemonchus contortus. The resistant isolates showed a tolerance (in terms of the concentration of compound required to reduce feeding to 50%) toward ivermectin of approximately 4.5- and 9-fold and toward ivermectin aglycone of approximately 14-fold, compared to the susceptible isolate. This indicates that susceptible and resistant isolates can be readily distinguished on the basis of the sensitivity of pharyngeal uptake to MLs. The use of various metabolic inhibitors in this assay system did not reveal the nature of the resistance mechanism. Pretreatment of resistant larvae with inhibitors of multidrug resistance mechanisms (P-glycoprotein and multidrug resistance protein) and detoxification enzymes (monooxygenases, esterases, and glutathione transferases) did not reduce their level of tolerance to the ivermectin aglycone.