Thresholds for detection of cryolite recovery sludge (CRS, an aluminium smelter waste dumped at sea) were determined for salmon. Salmo salar (L.). in sea water using a cardiac conditioning technique. Some effects of exposure of salmon to CRS filtrate (mean concentration 5.5° D) for up to 1 h were monitored. Oxygen consumption and ventilation rate increased and heart rate decreased after presentation of the filtrate. The effects were transient and the physiological parameters monitored returned to pre-exposure levels within 2 h of ceasing filtrate presentation. Detection levels for an amino acid (glycine) were determined for salmon in sea-water before and after exposure to CRS filtrate (mean concentration 5.5%). Within 24 h of exposure to the CRS filtrate the mean threshold level was significantly different from the pre-exposure detection level. However, 24–48 h after exposure the threshold for detection of glycine was no longer significantly different.