Resistance to the anthelmintics pyrantel ((E)-1,4,5, 6-tetrahydro-1-methyl-2-[2-(2thienyl)ethenyl]pyrimidine) and levamisole ((S)-2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]thiazole) is an increasingly widespread problem in gastro-intestinal nematode infestations. Both compounds act on the nicotinic acetylcholine receptors on the surface of nematode somatic muscle. The patch-clamp technique was used to measure nematode nicotinic acetylcholine receptor properties at 75, 50, -50 and -75 mV in a pyrantel-resistant isolate of Oesophagostomum dentatum. Patch pipettes contained 30 microM levamisole as agonist. We found that 28. 1% of membrane patches contained active receptors. At -50 mV, the single-channel conductance was 36.2+/-1.4 pS, the mean open-time (tau) was 1.45+/-0.14 ms and the mean probability of opening (P(o)) was 0.004+/-0.002. We compared these results with previous work on an anthelmintic sensitive isolate and a levamisole-resistant isolate [Robertson, A.P., Bjorn, H.E., Martin, R.J., 1999. Levamisole resistance resolved at the single-channel level. FASEB J. 13, 749-760.]. We found that pyrantel-resistant parasites had a reduced percentage of active patches and a reduced P(o) value when compared to anthelmintic sensitive worms. We concluded that pyrantel resistance is associated with a modification of the target nicotinic receptor properties.