Six-wk-old piglets of both sexes from European Large White (LW, n = 36) and Chinese Meishan (MS, n = 24) breeds were individually exposed to a novel environment, a stressful stimulation. Behavioral and pituitary-adrenal reactivity were investigated. When compared with LW, MS pigs displayed low locomotion (18.5 +/- 2.2 vs. 41.0 +/- 3.8 squares crossed/10 min; P < 0.0001), and defecation scores (0.58 +/- 0.15 vs. 4.86 +/- 0.37 fecal boli; P < 0.0001). Basal concentrations of cortisol were higher in MS (96.1 +/- 1.1 vs. 44.9 +/- 1.1 ng/ml; P < 0.0001), although no differences between breeds were found in basal concentrations of adrenocorticotropic hormone (ACTH). In response to novel environment exposure, the ACTH increase was greater in LW than in MS, but the cortisol response was not different on a log scale. To further investigate the pituitary-adrenal differences between the two breeds, the 24-hr profile of ACTH and cortisol plasma concentrations, a corticotropin-releasing factor (CRF) and a coupled dexamethasone-ACTH test were studied. Five castrated male 9-wk-old piglets from each breed were fitted surgically with a jugular vein catheter. A classic marked circadian rhythm of cortisol and a weak nycthemeral variation of ACTH were found. Cortisol concentrations were approximately twice higher in MS exclusively during the early light phase (from 0800-1200 hr) of the cycle, but no significant interbreed difference was found in the circadian rhythm of ACTH. Administration of CRF (1 microgram/kg iv) induced the same significant increase in plasma ACTH and cortisol concentrations in both breeds. Administration of ACTH (10 micrograms/kg i.v.) increased significantly cortisol concentrations and revealed no difference in plasma cortisol response to ACTH. These data suggest that the hypercortisolism of MS pigs is of adrenal origin, and related to extrapituitary factors that control the adrenal sensitivity during the light phase of the diurnal cycle.