A primary infection of a parasite naïve animal has detrimental effects on intake (anorexia), which in turn results in impaired animal productivity. Recent studies have indicated that anorexia in nematode infected lambs is a direct consequence of the acquisition of immunity (Greer et al., 2005). In many models of disease, immune system activation results in elevated leptin levels and these have been associated with anorexia. However, whether the expression of acquired immunity following a secondary nematode infection results in elevated leptin levels and/or anorexia in growing lambs it is not known. In addition, it is not known whether the expression of acquired immunity differs between breeds that differ in production potential. The aim of the present study was to test the hypotheses a) that a secondary nematode infection results in increased leptin levels and anorexia in growing lambs and b) that lambs of a high production potential breed exhibit a higher degree of anorexia than lambs of a low production potential.