Severe pathological changes were observed in the adrenal and thyroid glands of Zebu (Bos indicus) heifers infected with Trypanosoma congolense. In the adrenal glands, severe inflammatory changes characterised by mononuclear cellular infiltration in the subcapsular areas, zona glomerulosa, zona fasciculata and zona reticularis were observed. In addition, there were hyperaemia, haemorrhage and hyperplasia resulting in increased adrenal weight in the two heifers slaughtered on Days 50 and 70 post-infection (p.i.). In the medullary areas of these two heifers, there was only mononuclear cellular infiltration. In the two heifers slaughtered on Day 113, localised mononuclear cellular infiltration was observed in addition to cortical cell depletion and fibrosis, resulting in reduced adrenal weight when compared with the controls. In the thyroid glands of the infected heifers, gross enlargement of the follicles filled with pale staining colloids were observed in association with squamous metaplasia of the follicular epithelium and fibroplasia. Serum cortisol concentrations showed an appreciable but non-significant increase (P less than 0.05) between Weeks 2 and 6 p.i. in the infected heifers (5.6 +/- 0.86 ng ml-1 vs. 4.4 +/- 0.34 ng ml-1) when compared with the uninfected controls or the pre-infection level. Thereafter, cortisol levels declined though non-significantly throughout the rest of the study. However, there was a steady increase towards normal levels from Week 12 p.i. until the end of the experiment. Changes in the basal serum concentrations of thyroxine (T4) were also measured weekly. There was a progressive decrease in the levels of T4 from the third until the 11th week p.i., when the decrease became significant (P less than 0.05) and remained so until the 16th week when the experiment ended.