The pathogenesis and pathology of African animal trypanosomosis (AAT) in Baoulé, N'Dama/Baoulé-cross-bred and Zebu cattle was studied from 1987 to 1991 in a series of experiments conducted under natural and artificial conditions of challenge at the Centre de Recherches sur les Trypanosomoses Animales (CRTA) in Burkina Faso. This first paper reports on the clinical performance of 64 Baoulé, 10 N'Dama/Baoulé-cross-bred and 20 Zebu cattle, which were transferred to the pastoral zone of Satiri, 50 km northeast of Bobo-Dioulasso, a zone infested with Glossina palpalis gambiensis, G. morsitans submorsitans and G. tachinoides. Prior to the experiment, the cattle had been raised in a fly proof stable and at the CRTA breeding station, an area of extremely low incidence of trypanosomosis or had been exposed at least once to natural trypanosome challenge in an area of high Glossina density. The cattle were monitored daily for clinical performance. Blood samples were collected twice weekly and examined on the spot for packed red cell volume (PCV) and parasitaemia. In the blood of 98% of the cattle trypanosomes (Trypanosoma vivax, T. congolense) were detected. Significant inter- and intrabreed differences with respect to the clinical performance were recorded. Regarding general health, the humpless Baoulé and N'Dama/Baoulé cross-bred cattle (Bos taurus) proved to be superior to the humped Zebu cattle (B. indicus) under this high challenge. Previous exposure to natural challenge had a positive effect on survival for both Baoulé and Zebu cattle. The phenotypic variation in response to trypanosomosis was small in Baoulé previously exposed and large in Baoulé previously not exposed.(ABSTRACT TRUNCATED AT 250 WORDS)