Eighteen, six- to seven-month-old lambs were infected experimentally with larvae of Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus axei, Trichostrongylus colubriformis, Cooperia curticei and Nematodirus spathiger, and allocated to three equal groups. The infections were timed to ensure that fourth-stage larvae were present when groups 1 and 2 were treated orally with monepantel. Group 1 was not fed for 24 hours before the treatment, group 2 was fed two hours before the treatment and group 3 was fed at the same time as group 2 but not treated with monepantel. All the sheep had access to water. Worm burdens were determined 15 days after the treatments. Fasting or feeding had no statistically significant effects on the efficacy of the monepantel solution against the nematodes, and the period of fasting had no adverse effects.