Herbage intake was measured in two groups of 20 first-year grazing cattle. The animals in one group each received a morantel sustained release bolus at turnout to control nematode parasitism and the animals in the other group remained untreated. The latter group showed a mean peak faecal egg count of 655 eggs per gram (e.p.g.) in October associated with high serum pepsinogen concentration and clinical signs of ostertagiasis, compared with a peak of 119 e.p.g. in the treated group which remained in good health. In September the daily voluntary feed intake of the untreated animals was significantly depressed (94 g kg-1 body weight vs. 77 g kg-1 P less than 0.001), but no difference in digestive efficiency was recorded between the two groups. This difference in feed intake was associated with a 47 kg mean live weight advantage of the treated animals at housing.