Fig. 6. (A) Plasma [cortisol] of rainbow trout exposed to 0, 500, 750 or 1000 µmol l^-1 NH₄Cl for either 24 or 96 h periods. Values are means + 1 S.E.M. Treatments that do not share a common letter for a given time are significantly different from each other. ^*Significant differences between exposure periods for a given treatment (P<0.05; N=20-23). (B) Relationship between plasma [ammonia] and plasma [cortisol] after 24 h NH₄Cl exposure (r²=0.41, P<0.001). (C) Relationship between hypothalamic serotonergic activity and plasma [cortisol] after 24 h NH₄Cl exposure (r²=0.25, P=0.002). Serotonergic activity is expressed as a ratio of the metabolite 5-hydroxyindole acetic acid (5-HIAA) to serotonin (5-HT).