The requirements of metabolizable energy (ME) and metabolizable crude protein (MCP) of Merino sheep in response to infection with Trichostrongylus colubriformis and Ostertagia circiancincta were measured in an animal house environment. Two genotypes of growing sheep, parasite-resistant line and unselected control, were fed at maintenance, 1.5 x maintenance, or ad libitum for 24 or 18 weeks in two experiments, and the body weight, weight change, and wool growth rate of the sheep were measured at six weekly intervals. Extra protein required for infected sheep was 6.5 g/day for sheep at maintenance and about 9 g/day for sheep gaining 100 g/day weight, and was consistent across the body weight range. The extra energy requirement by the infection ranged from 0.71 to 1.76 MJ/day, depending on the body weight and weight gain, and declined with increasing body weight. The costs of MCP and ME by the infection in relation to total nutrient requirements declined with increasing body weight, indicating that the infection had more severe detrimental effects on young sheep. Compared with the control line, the parasite-resistant sheep at 10 months of age required an additional 0.02 MJ ME/kg of metabolic live weight (W-0.15) and 0.19 g/W-0.75 MCP. These extra requirements by the resistant sheep were approximately 4% of the daily ME and 5% MCP requirements in young sheep. At an older age (18 months), the additional nutrient requirement by resistant sheep was no longer evident. (c) 2005 Elsevier B.V. All rights reserved.