While several in vitro studies have shown that the anthelmintic ivermectin inhibits feeding by parasites, the relevance of this putative site of action in vivo has not been demonstrated. For this study, techniques to measure feeding by Haemonchus contortus in vivo relied on the blood feeding characteristics of the worm, and utilised tritiated inulin administered to sheep intravenously and subsequently measured in worms recovered from abomasa. Nematodes recovered from sheep treated with ivermectin 4 h prior to the [3H]inulin administration showed equivalent feeding levels (over a 1 h period) to those recovered from sheep not treated with ivermectin. In addition, there was no difference in the radioactivity in nematodes of an ivermectin-susceptible and an ivermectin-resistant isolate recovered from individual sheep with concurrent infections after a dose with ivermectin. Ivermectin, therefore, had no effect on feeding by H. contortus in vivo under these experimental conditions. The results are discussed in relation to the dynamics of the expulsion of H. contortus from sheep following ivermectin treatment.