An experiment was carried out on treatment of Trypanosoma brucei brucei experimentally infected sheep with central nervous system involvement with a 2-nitroimidazole derivative: Ro 15-0216 (Hoffmann La Roche & Co Ltd, Basel, Switzerland). After a single intramuscularly or intravenously administration (25 or 50 mg/kg) of this compound in healthy sheep, levels of more than 1 microgram/ml corresponding to trypanocidal concentrations are detected in both blood and cerebrospinal fluid. However, because of short half life, two or three injections a day appeared to be necessary. Six infected sheep were treated by daily intravenous injection for six days: three animals with 25 mg/kg/d and three others with 50 mg/kg/d. Animals which relapsed received 50 mg/kg/d divided in three intravenous injections a day for six days. After a year of observation, all the animals were considered clinically and biologically cured.