Alcohol ingestion activates the autonomic nervous system and the hypothalamic-pituitary-adrenal axis. This study examined naltrexone effects on alcohol-induced increases in physiological responses and their association with alcohol liking. Using a within-subjects design, heavy drinking men (N ϭ 19) were maintained on each of three naltrexone doses (0, 50, and 100 mg, p.o.) over an 8-day inpatient stay. Within each naltrexone dose, subjects had three alcohol challenge sessions (none, moderate, high) The brain opioid system has been proposed to be part of the neurocircuitry involved in alcohol reward and heavy alcohol drinking (Froehlich and Wand 1997;Gianoulakis and deWaele 1994;Reid et al. 1991). Pharmacological studies indicate that opioid receptor antagonists, such as naloxone hydrochloride and naltrexone, decrease alcohol self-administration in animal models (Altshuler et al. 1980;Froehlich et al. 1990;Hubbell et al. 1986Hubbell et al. , 1991Marfaing-Jallat et al. 1983;Samson and Doyle 1985;Weiss et al. 1990). Clinical trials demonstrated that naltrexone reduces alcohol drinking and self-reported "high," alcohol craving, as well as relapse rates in recently abstinent outpatient alcohol-dependent patients (O'Malley et al. 1992;Volpicelli et al. 1992Volpicelli et al. , 1995 . These findings led to naltrexone's approval by the Food and Drug Administration as a pharmacotherapeutic agent for the treatment of alcohol dependence. Naltrexone is thought to reduce alcohol-induced high and relapse by binding to opioid receptors in discrete brain regions associated with reward and craving, and to prevent receptor activation by alcohol-induced release of endogenous opioid peptides.Alcohol induces a variety of acute physiological responses when consumed in moderate-to-high doses. It is well established that intoxicating doses of alcohol increase heart rate (Docter et al. 1966;Iwase et al. 1995 McCaul et al. 1989;Sellers et al. 1972;Turkkan et al. 1988;van de Borne et al. 1997). These heart rate increases are hypothesized to be positively associated with the reinforcing or appetitive motivational effects of alcohol (Peterson et al. 1996) . Alcohol has less predictable effects on blood pressure, with reports of blood pressure increases (Iwase et al. 1995;Sellers et al. 1972) and no effects following alcohol ingestion (Turkkan et al. 1988;van de Borne et al. 1997). Finally, alcoholinduced peripheral vasodilation and associated skin temperature increases have been consistently reported (Iwase et al. 1995;Turkkan et al. 1988) and is thought to be one explanation for the lack of a consistent rise in blood pressure after alcohol consumption.Acute administration of intoxicating doses of alcohol also activates the hypothalamic-pituitary-adrenal (HPA) axis thereby increasing ACTH and glucocorticoid levels (Inder et al. 1995;Rivier et al. 1984;Waltman et al. 1993). It remains unclear whether alcohol-induced activation of the HPA axis has any relationship to its reinforcing properties. Rodent studies show that alcohol increases pla...