Softwater (Ca2+=50, Na+= 50(μequiv. l−1) acclimated rainbow trout were fitted with chronic arterial catheters to allow for repetitive blood sampling. After 48 h recovery they were then exposed to either control (pH 6.5, Al = 0μg l−1), acid (pH 4.8, Al = 0μg l−1) or acid plus aluminum (pH 4.8, A1 = 112 μg l−1) conditions for 72 h. Parameters measured included blood glucose, lactate, haemoglobin, haematocrit and plasma Na+, Cl−, protein and cortisol.Exposure to pH 4′8 alone caused no mortality, a moderate ionoregulatory disturbance and a transient elevation in plasma cortisol. All other parameters were not significantly different from controls. Addition of aluminum to this exposure caused 100% mortality with a mean survival time of only 27.0 h. There was a marked decrease in plasma ions, hyperglycemia, lactate accumulation, haemoconcentration, red cell swelling, and a sharp rise in plasma cortisol becoming greatly increased as the fish neared death. The mechanism of toxicity of acute acid/aluminum exposure, the role for cortisol under such conditions, and the validity of cortisol and glucose as indicators of stress in fis