Exposure of sexually mature pre-spawning Atlantic salmon Salmo salar to Fossbekk water (pH 5粩 for 7 days led to a significant reduction in critical swimming speed (U-crit) in females but not in males. Exposure to Fossbekk water +A1 (as AlCl3) for 24 h led to a significant reduction in U-crit in both males and females. In contrast to fish exposed for 7 days to Fossbekk water, fish exposed to Fossbekk+A1 had accumulated much more aluminium and mucus on their gills. Losses of plasma ions were similar in both groups exposed to acid water. Blood glucose was twice as high in fish exposed to Fossbekk water for 7 days compared with fish exposed to Fossbekk+A1 for 24 h. Plasma cortisol was still elevated compared with controls after exposure to Fossbekk water for 7 days. Possible mechanisms for the observed decrease in U-crit at the different exposures are discussed.