Striped bass (Morone saxatilis) exposed to a standardized confinement stress had markedly different clinical and endocrinologic responses, compared with hybrid striped bass exposed to the same stress. Plasma cortisol concentration increased at a faster rate and appeared to reach a higher value in striped bass than in hybrid bass. Mean plasma cortisol concentration was 742 +/- 43 ng/ml in striped bass, compared with 490 37 and 531 +/- 40 ng/ml in striped bass x white perch (M americana) and striped bass x white bass (M chrysops) hybrids, respectively, after a 45-minute net confinement. Plasma cortisol concentration also remained significantly (P = 0.003) higher in striped bass for at least 48 hours after the net confinement. These hormonal differences were associated with a markedly lower survival and resistance to infection in striped bass, compared with the hybrids.