The role of endogenous opioid mechanisms in the pituitary-adrenocortical response to acute stress was investigated in a longitudinal study in cyclic female pigs before and after exposure to chronic stress (long term tethered housing). Challenge of loose-housed pigs with acute nose-sling stress for 15 min induced an activation of the hypothalamic-pituitary-adrenocortical axis, evidenced by a transient increase in plasma ACTH (peak height above basal, 98 +/- 12 pg/ml; mean +/- SEM) and cortisol (54 +/- 3 ng/ml) concentrations. Pretreatment with the opioid receptor antagonist naloxone (0.5 mg/kg BW, iv bolus) increased the challenge-induced ACTH and cortisol responses to 244 +/- 36 pg/ml and 65 +/- 5 ng/ml, respectively. This indicates that during acute nose-sling stress, endogenous opioid systems are activated that inhibit the pituitary-adrenocortical response. After exposure of the pigs to chronic stress (10-11 weeks of tethered housing), the challenge-induced ACTH response was attenuated, whereas the cortisol response remained unchanged, suggesting an increased adrenocortical sensitivity to circulating ACTH. In addition, pretreatment with naloxone induced a greater increment in the ACTH and cortisol responses in tethered pigs than in loose-housed pigs. As no such changes were found in control animals housed loose during the entire experimental period, this indicates that the impact of opioid systems had increased due to chronic stress. The increased impact of opioid systems during chronic stress may prevent excessive hypothalamic-pituitary-adrenocortical responses to acute stressors and, thus, may be of adaptive value.