Endogenous opioid activity has been associated with the regulation of mood and inhibition of the hypothalamicpituitary-adrenal (HPA) axis. We assessed differences in psychological symptomology and naloxone sensitivity in non-alcoholic males and females with a family history of alcoholism (FHP) and without a family history of alcoholism (FHN).Family history of alcohol dependence is a strong predictor of risk for the future development of alcoholism (Goodwin 1985;Cloninger 1981;McGue et al. 1992). Adult sons of alcoholic fathers have a three to four-fold higher risk for this disorder compared to the male offspring of non-alcoholic fathers (Schuckit 1981).These findings have led investigators to examine the presence of a wide range of possible psychological as well as neurobiological mediators for the development of alcoholism in individuals with a positive family history (FHP) for alcoholism (Pihl et al. 1990;Martin and Sher 1994;Baker and Stephenson 1995). A wide range of psychological differences have been specifically associated with FHP males relative to controls, including impulsivity (Saunders and Schuckit 1981;Schulsinger et al. 1985), conduct disorder (Nylander and Rydelius 1982), antisocial personality with or without hyperactivity (Alterman et al. 1983), attention deficit disorder (Gillen and Hesselbrock 1992;Deckel et al. 1995), and aggressiveness (Windle 1990 NO . 6 family history for alcoholism compared with normals (Newlin and Thomson 1990;Schuckit et al. 1988Schuckit et al. ,1996Waltman et al. 1994).A growing body of literature suggests that the endogenous opioid system modulates a wide range of psychological symptomology, as well as the functioning of the Hypothalamic-Pituitary-Adrenal (HPA) axis (Pickar et al. 1982;Olson et al. 1996). It has been speculated that the interplay between environmental and genetic determinants generates a spectrum of endogenous opioid activity in the human population which, in the extreme ranges of opioid expression, lead to disturbances in mood, behavior and neuroendocrine function (Wand et al. 1998;Wand 1999;Blevins et al. 1994). Major behavioral alterations have been induced by opioid blockade with naloxone, suggesting the involvement of the endogenous opioid system in the tonic regulation of mood, behavior and cognition in normal (Cohen et al. 1983;Martin del Campo et al. 1992) and psychiatric populations (Insel and Pickar 1983;Sandyk 1987;Pickar et al. 1982). Some clinical studies suggest that in addition to the serotonergic system, the opioidergic system may be implicated in obsessive-compulsive symptomology (Carrion 1995;Insel and Pickar 1983;Keuler et al. 1996;Rapoport et al. 1992). Plasma B-endorphin levels have been found to be significantly lower in OCD subjects than in controls (Weizman et al. 1990). Studies with OCD subjects have shown naloxone decreases obsessional doubt and compulsive behaviors at low doses (Insel and Pickar 1983;Keuler et al. 1996) and decreases at higher doses (Sandyk 1987) in a small subgroup of subjects. Furthermore, opioid agent...