Twenty dogs of mixed local East African breeds were used. Five of the dogs were uninfected controls and 15 were infected with T. brucei (ILRAD 273). Five of the infected dogs were untreated controls, five were treated with a high curative dose of diminazene aceturate, (7 mg kg-1 body weight (wt.), and five were given a subcurative dose of isometamidium chloride (1 mg kg-1 body wt.). The drugs, given at 8 days post infection (d.p.i..), led to apparent recovery. The antibody titres, however, remained high in both groups and at 42-49 d.p.i. there was at least one relapse in each treatment group. Parasite populations from relapsed animals were more resistant to the drugs than the original infecting populations. The implications of these findings are discussed.