Gambel's white-crowned sparrows were captured and brought into captivity in order to study seasonal changes in the function of the hypothalamic-pituitary-adrenal (HPA) axis in captive birds. 30 min of restraint elicited a rise in corticosterone titers that varied depending upon the season and physiological state of the birds. Restraint elevated corticosterone titers significantly more during the fall (within 2 weeks of capture from the wild) than during either the winter or during a prealternate or prebasic molt. We also examined what changes in the HPA axis could account for altered corticosterone levels. Exogenous ACTH significantly elevated corticosterone levels beyond the response to restraint during the fall, indicating a dramatic enhancement of the adrenal's ability to secrete corticosterone. Exogenous ACTH was ineffective at other times, suggesting that the adrenal's ability to release corticosterone often limits circulating levels. We further inferred the pituitary's ACTH secretory ability by injecting exogenous corticotrophin-releasing factor, arginine vasotocin, and mesotocin and measuring corticosterone release. Pituitaries failed to respond to any exogenous releasing factor during the fall, suggesting that the pituitary may be the site in the HPA axis regulating corticosterone release at this time. When compared to wild-caught birds, these results suggest that captivity alters both adrenal and pituitary function during restraint in white-crowned sparrows, and that this change depends upon the season and/or physiological state of the animal. Captivity thus appears to have a profound affect on the function of the HPA axis, and these results reiterate the caution that must be used to extrapolate laboratory data to field conditions.