The mode of action of ivermectin (IVM) in nematodes appears to be the opening of inhibitory ion channels, including the glutamate-gated chloride channel (GluCl). Recently, it has been shown that IVM binds with high affinity to a Haemonchus contortus GluCl subunit (HcGluCla) expressed in COS-7 cells, and this binding is potentiated in the presence of glutamate. To gain further insight into the potentiation of macrocyclic lactone anthlemintics we have screened various glutamatergic and nonglutamatergic ligands for their ability to enhance [ 3H ] IVM binding to HcGluCla. Of the ligands tested, only glutamate and ibotenate potentiated [ 3H ] IVM binding. Interestingly, these ligands have also been shown to open the HcGluCla channel expressed in Xenopus oocytes. We examined the effect of ibotenate on macrocyclic lactone binding in more detail and found that it caused a 7-fold enhancement in [ 3H ] IVM binding affinity and a 4-fold increase in [ 3H ] MOX binding affinity. In in vivo efficacy studies, ibotenate (up to 2mg/kg) had no anthelmintic activity against H. contortus in gerbils. When 1mg/kg ibotenate was used in combination with IVM, IVM efficacy increased by 15% ( P=0.048 ). These results demonstrate that a GluCl agonist enhances IVM activity and provides further information on the mode of action of ivermectin in parasitic nematodes.