We investigated the effects of naltrexone (NTX) on alcohol drinking, urge to drink alcohol, and alcohol-induced sensations and mood states in social drinkers consuming alcohol ad libitum in a cocktail bar. Sixteen college-age men and women participated in a double-blind, placebo-controlled, within-subjects, cross-over study. Subjects were tested during each of three drug conditions: NTX, 50 mg/ day, po; inactive placebo; and no drug. Each treatment condition lasted 8 to 11 days. Small groups of subjects consumed alcohol ad libitum during three 2-hr evening drinking sessions, separated by approximately-2 weeks. NTX treatment significantly increased the latency (time in seconds) to first sip the first (p < 0.05) and second alcoholic beverages consumed (p < 0.01). Moreover, the mean blood alcohol concentration at the end of the session was significantly lower when subjects were treated with NTX (p < 0.05). No differences were found on self-report urge to drink alcohol. Subjects reported more fatigue and tension on the Profile of Mood States (p < 0.05), before drinking, and increases in nausea on the Alcohol Sensation Scale (p < 0.05) when treated with NTX. The increase in the latency to sip the first and second alcoholic beverages may reflect the capacity of NTX to block urge for alcohol elicited from external cues (before consuming alcohol), as well as urge for alcohol after priming from ingested alcohol. Thus, the effectiveness of NTX for reducing drinking behaviors of alcoholics may be partially caused by anticraving properties of NTX.