We investigated the effect of chronic nicotine self-administration (SA) on hypothalamic-pituitary-adrenal (HPA) hormonal responses to acute stressors. Adult male Sprague-Dawley rats were given access to nicotine (0.03 mg/kg) for 23 h per day for 20 days. On day 1 of acquisition of nicotine SA, plasma levels of both adrenocorticotropin and corticosterone were significantly increased 15-30 min after the first dose of nicotine. These hormonal changes were no longer significant on day 3, when adrenocorticotropin levels were <60 pg/ml and corticosterone levels were <110 ng/ml during the hour after the first dose of nicotine. Chronic nicotine SA (20 days) significantly augmented (2-3-fold) both hormonal responses to mild foot shock stress (0.6 mA, 0.5 s per shock, 5 shocks per 5 min), but did not affect hormonal responses to moderate shock (1.2 mA, 0.5 s per shock, 5 shocks per 5 min), lipopolysaccharide or immobilization. Similar data were obtained in Lewis rats. These results provide further support for the concept that chronic nicotine SA is a stressor. In alignment with the effects of other stressors, nicotine activated the HPA axis on the first day of SA, but desensitization occurred with repeated exposure. Furthermore, chronic nicotine SA selectively cross-sensitized the HPA response to a novel stressor. These observations suggest that nicotine may selectively increase the HPA response to stressors in human smokers.