5,7-Dihydroxytryptamine (5,7-DHT) or the vehicle was administered once into both lateral ventricles of the rat. Desmethylimipramine (DMI) was administered IP prior to the intraventricular injection of 5,7-DHT to prevent the destruction of norepinephrine (NE) terminals. Following recovery from surgery, ethanol (5 g/kg, PO) or isocaloric sucrose was given daily for 25 days. Tests at 5-day intervals showed that chronic ethanol treatment produced tolerance to the motor impairment on the moving belt test and to hypothermic effects of ethanol. The 5,7-DHT treatment did not alter either the motor impairment or hypothermia produced by the initial dose of ethanol. However, 5,7-DHT treatment produced a 75% depletion of brain serotonin (5-HT) without altering NE concentration and retarded the development of tolerance to ethanol in both measurements. This study with a specific central depletor of 5-HT, without alteration in NE concentration, extends and supports our hypothesis that brain 5-HT modulates the development of tolerance to ethanol.