Acetaldehyde, the most toxic metabolite of ethanol, has been implicated in many toxic effects of ethanol. In the present study, we used rats to investigate the possible changes on the sleep-wake cycle and brain regional concentrations of noradrenaline and serotonin after intraperitoneal administration of several doses of acetaldehyde. Results showed that acetaldehyde significantly decreases the time spent in rapid eye movements sleep and wakefulness and increases the time spent in slow-wave sleep. The neurochemical analysis showed that acetaldehyde significantly increases the 5-hydroxyindolacetic acid content and 5-hydroxyindolacetic acid/serotonin ratio at the bulb and pons. These results suggest that acetaldehyde modifies the metabolism of serotonin which has been implicated in the onset and sustaining of the sleep episodes occurring along the sleep-wake cycle.