The suggestion that acetaldehyde may be endowed with positive reinforcing properties and may in fact mediate some of the psychopharmacological actions of ethanol has been examined by us and other investigators using a variety of paradigms. We first reported that non-dependent animals would self-administer acetaldehyde through an intra-cerebroventricular route. In addition, we have demonstrated that central infusion induced a conditioned place preference. We have also shown that an animal's propensity to self-administer acetaldehyde directly into the brain was related to its subsequent voluntary intake of ethanol. Lastly, we have reported that inhibition of acetaldehyde metabolism resulted in an enhancement of alcohol-induced euphoria in man. The data collected to date from four different paradigms strongly support the notion that acetaldehyde is endowed with positive reinforcing properties which may play a critical role in the mediation of ethanol euphoria.