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Higher corticosterone (CORT) responses to acute stress have previously been reported in quail selected for short (STI) duration of tonic immobility (TI) than for long TI (LTI), although behavioral studies indicated that LTI quail were more fearful. To investigate adrenal and pituitary function in these quail lines and their possible involvement in the differences in hypothalamic-pituitary-adrenal (HPA) axis reactivity, we measured CORT responses to adrenocorticotropin (1-24 ACTH), corticotropin-releasing factor (CRF), and arginine vasotocin (AVT) after characterizing the nucleotide acid sequences of these peptides in quail. Although maximum adrenal responses, assessed by ACTH challenge, were higher in STI quail, adrenal sensitivity was comparable for the two genotypes. It is therefore unlikely that differences in HPA axis reactivity involved the adrenal level. AVT and ACTH induced comparable CORT responses in both genotypes, whereas those induced by CRF were much lower. AVT is thus more potent than CRF in quail, but the respective maximum pituitary capacity of both genotypes to secrete ACTH was similar, and it is doubtful that the AVT pathway is involved in the difference in HPA axis reactivity between genotypes. On the other hand, the higher CORT responses induced by CRF in STI quail suggest that CRF might be involved in the differences in HPA axis reactivity between LTI and STI genotypes. hypothalamic-pituitary-adrenal axis; corticosterone; stress