The physiological responses of juvenile American shad Alosa sapidissima were evaluated during the period of downstream migration in freshwater (FW) and after seawater (SW) acclimation associated with postmigration. Fish were subjected to a standardized, acute handling and confinement stress (3 h). Changes in plasma cortisol, plasma chloride, and hematocrit were monitored for 24 h. Basal levels of plasma cortisol were 5 times as great in FIX as in SW fish (34 and 7 ng/mL. respectively). Within 0.5 h. both groups exhibited Significant increases in plasma corti ol, The increase in FW fish was 4.5 times as great as that in SW fish. While the cortisol levels in SW fish returned to their basal values within 24 h, those of FW fish remained more than 25 times as high as basal values. Changes in plasma chloride occurred in fish after the initial stress, decreasing in FW fish and increasing in SW fish, This perturbation was overcome within 24 It in SW fish but not in FW fish. Hematocrit increased in FW fish 3 h after the initial stress and returned to normal within 24 h; fish stressed in SW exhibited no change in hematocrit. Significant mortalities were observed in the FW group but not in the SW group. During the period of downstream migration in FW, fish exhibited a heightened sensitivity to acute stress compared with that of SW-acclimated post-migrants. Handling and confinement associated with fish transport and fish passage structures are therefore likely to impact the performance scope and survival of juvenile American shad