Morphine increases intake of beer among rats.

Rats were maintained on a daily regimen of 22 h of water deprivation followed by a 2-h opportunity to take water and sweetened alcoholic beverage containing 12% ethanol. After 30 days, the alcoholic beverage was changed to beer containing either 3% or 6% ethanol. After 20 daily sessions with beer, they received, before the next session, an injection of saline. On the next day, they received a 1.0 mg/kg injection of morphine before the session. Morphine reliably increased rats' mean intake of both kinds of beer. Subsequently, the concentration of ethanol in each groups' beer was changed. The 3% group's beer was switched to 6%, and the 6% group's to 3%. Both groups altered their mean intake of beer in an apparent attempt to maintain intakes of nearly the same amount of ethanol, but presentation of 6% beer resulted in greater intakes of ethanol.