Rats infected with the parasite Trypanosoma brucei brucei showed selective changes of c-fos expression in the suprachiasmatic nucleus of the hypothalamus (SCN) during spontaneous sleep (S) and wakefulness (W) under a basal 12 h/12 h light-dark (L-D) cycle. In the vast majority of W (D phase) control animals the SCN was devoid of cells displaying Fos-related immunopositivity, while Fos-like-immunoreactive (ir) neurones were detected in most S (L phase) control rats. In most infected animals, on the other hand, Fos-ir neurones were detected in the SCN during W, but not during the S period, with a significant difference between control and infected S rats. Thus, these data indicate that the basal c-fos expression in the SCN during the L-D and S-W cycles is considerably altered in experimental trypanosomiasis. This is the first observation of a selective change in the SCN in trypanosome-infected rat brains. Since the SCN plays an important role as a pace-maker for biological rhythms, this finding may provide a basis for understanding the pathogenesis behind endogenous rhythm dyregulation and changes in sleeping pattern in human trypanosomiasis (African sleeping sickness).