Aims. To determine the incidence of ruminoreticulum bypass in lambs before and after weaning, following drenching with anthelmintic-equivalent volumes of fluid; to investigate the influence of withholding feed for 24 hours before drenching on the incidence of ruminoreticulum bypass; and to investigate the hypothesis that ruminoreticulum bypass occurs more frequently following drenching with large than with low volumes. The relationship between these factors and the efficacy of benzimidazole anthelmintic drenches has been widely promoted in New Zealand and Australia.