In the present study, we sought to examine the expression of the c-fos proto-oncogene in the cat brain after two different procedures of 24 h sleep deprivation. A first group of cats was gently sleep-deprived; they were awoken by a gentle touch of the hand (n = 5). A second group was sleep-deprived by the water tank technique which is a stressful deprivation (n = 4). A third group was placed 2 h on the water tank and was therefore stressed but not sleep-deprived (n = 2). A fourth group (control group of basal and unspecific Fos expression) was not sleep-deprived (n = 5). These four groups allowed us to separate Fos expression due to stress from Fos expression due to sleep deprivation. On the one hand, compared with controls cats, an important increase in Fos expression, detected by immunohistochemistry, was observed in the preoptic area of sleep-deprived cats by both gentle and stressful methods. On the other hand, there was a significant increase in Fos expression in the lateral hypothalamus of gently deprived cats as compared with control cats. These data indicate that c-fos expression can be employed as a marker of some putative homeostatic mechanism regulating sleep. The only sites in which there was a significant increased number of c-fos expressing neurons were located in the preoptic area which is known to be involved in sleep and in the lateral hypothalamic area.