Brook trout (Salvelinus fontinalis) adapt to chronic sublethal acid/Al stress. The accompanying acclimation confers greater resistance to short-term increases in Al and acidity. Adult trout were exposed in flowing soft water to eight combinations of pH (6.5, 5.2) × Ca2+ (25, 400 μequiv/L) × Al (0, 75, 150 μg/L = 0, 2.8, 5.6 μmol/L). After 10 wk, blood sampling by caudal puncture revealed no significant variations in osmolality, plasma protein, or hemoglobin and only minor differences [Formula: see text] in plasma Na+ and Cl−. Overall, most electrolytes were higher in fish exposed to higher water Al and/or Ca2+; only plasma Ca2+ was directly depressed by low pH. Hematocrit was raised by both low pH and elevated Al. When trout naive to both acid and Al were challenged with pH = 4.8, Al = 333 μg/L under flow-through conditions, there were large negative whole-body Na+ fluxes and marked depressions of plasma Na+ and Cl−, hemoconcentration, and substantial mortality over 48 h. Prior exposure for 10 wk to pH = 5.2 plus either 75 or 150 μg Al/L prevented mortality and ameliorated or abolished these effects through a more rapid recovery of net Na+ balance. Prior exposure to pH = 5.2 alone ameliorated these effects only slightly.