AbstractSignificant changes in bluegill sunfish ventilation were observed at chronically toxic levels of various substances in under 1 h of exposure and usually within 15 min of exposure. A different ventilation response was observed between the heavy metal compounds and the chlorinated hydrocarbon substances. The response to metals zinc and cadmium was characterized by: (a) decrease in signal amplitude and (b) increase in Type I (large volume) coughs or gill purges. The response to chlorinated hydrocarbons was characterized by: (a) increase in erratic movement, (b) increase in gill purges and (c) increase in ventilatory frequency. Our results suggest that fish ventilatory monitoring can provide a powerful tool for rapidly assessing sublethal as well as acute toxicity on a real‐time basis. Furthermore, the type of response generated may be indicative of the type of toxicant in the environmen