Thirty-two N'Dama heifers were offered ad libitum Andropogon hay plus 10·2 g/kg liveweight (LW) groundnut hay (GNH) (L) or 10·2 g/kg LW GNH and 3·9 g/kg LW groundnut cake (GNC) (H). After 4 weeks on the diets, half of each group were infected intradermally with Trypanosoma congolense clone (ITC 50) (LI and HI). Peak parasitaemia occurred 6–8 days after infection and started to decrease c. 56 days later. No differences in parasitaemia were observed between LI and HI animals. Packed cell volume (PCV) fell in all treatments (by 5·4, 13·8, 3·7 and 9·4 units after 49–63 days post-infection (p.i.) for the L, LI, H and HI groups respectively) and significant effects of infection and diet were observed. GNH and GNC intakes were maintained during the trial; however, infected animals had a decreased intake of Andropogon hay. LI animals lost significantly more weight during the experimental period than the non-infected controls (−71·4 v. −13·7 g/day). Meanwhile, HI animals gained less weight compared with the H group (52·2 v. 167·6 g/day). Weight losses appeared to be due to decreased food intake. In the period 54–68 days p.i., plasma concentrations of albumin were lower and plasma protein was higher in infected animals. Plasma cholesterol concentrations were also lower in infected animals 54–68 days p.i. Plasma urea concentrations were higher in supplemented animals but were not affected by infection. The results showed that animals on a higher plane of nutrition showed less severe clinical signs of infection. However, for all the parameters considered, the magnitude of the difference between groups on different diets was similar for both infected and control animals, suggesting that mechanisms of resistance were not affected by the planes of nutrition considered.