We used captive house sparrows (Passer domesticus) to identify regulatory mechanisms underlying seasonal (mimicked by changes in photoperiod) and diel differences in corticosterone output. Corticosterone responses were measured during three simulated seasons: short-day and long-day photoperiods and while birds underwent a pre-basic molt. Under all three conditions we tested for adrenal sensitivity by injecting exogenous ACTH, for pituitary sensitivity by injecting corticotropin-releasing factor (CRF) and arginine vasotocin (AVT), and for diel changes by repeating the injections during the day and at night. The daytime adrenal sensitivities were greatest on long days, lower on short days, and lowest during molt. These data suggest that reductions in either adrenal sensitivity to ACTH and/or capacity to secrete corticosterone could explain lowered endogenous corticosterone titers during molt. Furthermore, adrenal sensitivity to ACTH and pituitary sensitivity to AVT appeared to be greatest at night. This suggests that both the adrenal's sensitivity to the ACTH signal and the pituitary's capacity to secrete ACTH might provide a mechanism allowing for diel changes in corticosterone titers. This differs substantially from what is known about diel regulation in rodents. Taken together, these data provide further evidence that there are complex regulatory mechanisms controlling diel and seasonal changes in corticosterone titers in birds.