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Abstract. Changes in plasma T4levels were investigated in prepubertal and adult female goats during the course of an experimental Trypanosoma congolense infection. A significant decline in the T4levels was observed within 1 week of trypanosome challenge. The levels remained low up to the end of the 7th weeks when prepubertal goats received a trypanocidal treatment, whereafter the values started to rise to normal pre-infection values. The post-infection plasma T4values in the untreated adult goats did not return to normal except in one resistant goat which recovered naturally after 16 weeks of the infection. The thyroid glands of chronically infected and untreated adult goats revealed marked atrophy, fibrosis and colloidopathy. It is concluded that trypanosomiasis rapidly impairs thyroid gland function in susceptible animals, but these changes can be reversed by early trypanocidal treatment before marked degenerative changes become evident or following self cure in resistant animals.