P-glycoproteins (P-gps) play an important role in the sensitivity of nematodes to anthelmintic drugs. They have been implicated in a number of anthelmintic resistances, particularly for macrocyclic lactone drugs. Hence, inhibition of nematode P-gps has been suggested as a means of reversing some types of anthelmintic resistance. The present study aimed to investigate the ability of the most-recently developed group of P-gp inhibitors (the so-called 'third generation' of inhibitors) including tariquidar, zosuquidar and elacridar, to increase the sensitivity of Haemonchus contortus larvae to various anthelmintics (ivermectin, levamisole and thiabendazole) in vitro. We compared these compounds to some older P-gp inhibitors (e.g. verapamil and valspodar). Larval migration and development assays were used to measure the sensitivity of larvae to anthelmintics alone, or in combination with P-gp inhibitors. Significant increases in sensitivity to ivermectin were observed with zosuquidar and tariquidar in larval migration assays (synergism ratios up to 6-fold). Several of the inhibitors increased the sensitivity of both the drug-resistant and -susceptible isolates (e.g. tariquidar with ivermectin in migration assays, zosuquidar with ivermectin in larval development assays), while others had significant effects on the resistant isolate only (e.g. zosuquidar with ivermectin in migration assays, verapamil with ivermectin in development assays). This suggests that some of the inhibitors interact with P-gps representing intrinsic pathways present across nematode populations with quite different drug sensitivities, while other inhibitors interact with P-gps of significance only to resistant nematodes, and hence most likely representing an acquired resistance mechanism. The study highlights the potential of the third generation of P-gp inhibitors for increasing the sensitivity of nematodes to anthelmintics.