The age-dependent effect of the specific 5-HT2 receptor antagonist ketanserin at two doses (1 mg/kg and 6 mg/kg) on six sleep-waking phases was electroencephalographically (EEG) examined in young, middle-aged, and old male Wistar rats. Because 5-HT2 receptor binding sites decrease with aging, the question of whether ketanserin can produce any effect on sleep-wake in advanced age was addressed. Ketanserin enhanced deep slow wave sleep in the three age groups. However, in the old rats this effect was much less pronounced and did not depend on the dose. Only in the young and middle-aged animals, ketanserin reduced wakefulness in a dose-dependent manner. The suppression of paradoxical sleep with ketanserin did not depend on the age. The results suggest that the model of aging may be useful to study the functional role of 5-HT2 receptors in sleep-waking regulation.