1. Ten 5-month-old lambs (29 (SE 1.2) kg), reared parasite-free and prepared with rumen duodenal and ileal cannulas, were paired and given rations of Ruminant Diet AA6 (90 g/kg live Weight0.75) by means of continuous feeders. From 6 months of age one of each pair was dosed daily with 2500 Trichostrongylus colubriformis larvae for 14 weeks. Untreated animals received the amount of ration consumed by their infected pair-mates the previous day.
2. During three periods, ((1) the week before and the first 2 weeks of dosing with infected larvae, (2) during weeks 5–7 and (3) during weeks 11–13 of dosing) all lambs underwent a series of experiments to determine their nitrogen balance, the amounts of N leaving the small intestine, the amount of 51CrC13-labelled plasma protein leaking into the small intestines, and the disappearance of 35S-labelled bacteria from the small intestine.
3. The infection caused varying degrees of feed refusal in all infected animals. As a result the values for N balance and for the flow of N at the ileum during the latter two periods were regressed against dry-matter intakes for each group in each period.
4. The infection caused a reduction (P < 0.05) in N retention and increased (P < 0.05) flow of N at the ileum. The increase in N flow at the ileum of infected lambs was greater (P < 0.01) at weeks 11–13 of dosing (infected–control 3.6 g N/d (standard error of difference (SED) 0.57), P < 0.01) than at weeks 5–7 of dosing (infected–control 1.5 g N/d (SED 0.57), P < 0.05).
5. There were no between-treatment or between-period differences in the disappearance of 35S-labelled bacteria from the small intestines of infected or control lambs, but the infection did cause an increase in plasma N leakage during both periods. During weeks 5–7 and 11–13, plasma N leakage in infected lambs was 1.1 g N/d (P < 0.01) and 1.7 g N/d (P = 0.056) respectively higher than that in the control lambs.
6. A proportion of the endogenous secretions which enter the small intestine is likely to be resorbed before the ileum. It was calculated that to account for the extra non-ammonia-N (NAN) flow at the ileum up to 3–5 g NAN/d during weeks 5–7 of dosing and 15–20 g N/d during weeks 11–13 of dosing could have entered the small intestine as mucin and sloughed cells.
7. The results seem to indicate that the nutritional penalty associated with the development of resistance to infection is greater than that associated with the primary infection.