Triptosine is a new L-5-hydroxytryptophan derivative whose effect has been studied in the Long-Evans alcohol-preferring rat. At an oral dose of 100 mg/kg once daily, triptosine reduced alcohol consumption by 42% in the second week of treatment and increased that of water by 80%. The results suggest that this precursor of serotonin might play an important role in diminishing preference for alcohol and reaccustoming the animal to water, without exerting an anorexic effect.