TNB (1,3,5-trinitrobenzene) is a by-product of the TNT (2,4,6-trinitrotoluene) manufacturing process and is also formed by the photolysis of TNT in natural waters. The effects of TNB on the ventilatory patterns and whole-body involvement rates of bluegills (Lepomis macrochirus) were determined. Fish were exposed for six days to concentrations of TNB ranging from 6% (0.03 mg/L) to 108% (0.61 mg/L) of the 96-h LC50. The lowest TNB concentrations causing significant changes in the parameters monitored were 0.13 mg/L (ventilatory depth) and 0.61 mg/L (cough rate and body movement). These responses occurred within the first 4 h of exposure. No effects on ventilatory rate were found. The large differences is sensitivity between the parameters monitored indicate that automated water or wastewater toxicity monitoring systems that utilize fish ventilatory or movement responses should use several end points to determine the presence or absence of toxicity. The most sensitive, short-term ventilatory changes measured in this test occurred at concentrations near the estimated chronic toxicity levels of TNB for fish. The range between the highest TNB concentration causing no responses and the lowest concentration causing short-term ventilatory responses was 0.06 to 0.13 mg/L. This is comparable to reported no effect/effect ranges for TNB in early life stage tests of 0.08 to 0.12 mg/L for fathead minnows and 0.08 to 0.17 mg/L for rainbow trout.