The study demonstrated that repetitive, intermittent exposures to aluminum (Al) under acidic conditions produced greater cumulative mortality and lower growth rates of brook trout than would be extrapolated from continuous exposure experiments. During a 24-d study, exposures were at pH 4.4 or 4.9 with 0.2-1.2 mg Al-L-1. Data were analyzed based on peak and 24-d mean Al concentrations of continuous and intermittent (2-d toxicant, 4-d ambient condition cycle) exposures. Survival was lower in groups exposed at pH 4.9 than those at 4.4 when intermittently exposed but not when continuously exposed. Growth rates were less for fish continuously exposed at pH 4.4 compared with those at pH 4.9, while intermittent exposures to Al acidity alone did not affect growth. Fish intermittently exposed to Al at pH 4.4 grew faster than those at pH 4.9, indicating greater Al toxicity at the higher pH. (Copyright (c) Canadian Journal of Fisheries and Aquatic Sciences, Vol. 43, 1986.)