A study was undertaken to assess whether the persistent anthelmintic effect of ivermectin in cattle would allow an extension of the drenching interval in the currently recommended preventive drenching programme for the control of gastrointestinal nematode infection in dairy calves. A comparison was made of the parasitological and production responses of treatment groups of calves, grazing replicated plots, receiving either six drenches of oxfendazole at four-weekly intervals or four subcutaneous treatments with ivermectin at six-weekly intervals. Compared with the levels of infective larvae on pasture grazed by untreated control calves, mean larval numbers on pasture grazed by ivermectin and oxfendazole treated calves were 94.3% and 98.3% lower, respectively, during the period of maximum larval availability (March-May). Mean liveweight gains (December-August) of the treated groups (101.4kg and 110.2kg respectively) were not significantly different, but both were highly significantly different from that of the untreated controls (57.4kg). Mean plasma pepsinogen levels for the ivermectin, oxfendazole and control groups over the period of maximum values (June-August) were 1.92 i.u., 1.72 i.u. and 5.53 i.u., respectively. The difference between the treated groups was not statistically significant but both were highly significant different from the control group. The present results indicate that four treatments with ivermectin (subcutaneously) at six-weekly intervals achieved a similar level of prophylactic control to that effected by six treatments with oxfendazole at four-weekly intervals.