Fathead minnows (Pimephales promelas) were exposed to acutely toxic concentrations of organic chemicals under flow-through conditions at 25°C for 96 h. Changes in behavior and morphology were systematically recorded to develop a plan for classifying these chemicals. On this basis, three behavioral toxicity syndromes were evident. Each syndrome appears to represent a different general mode of toxic action. The hypoactivity syndrome, reflecting narcosis, is characterized by depressed locomotor activity, loss of startle response and very dark body coloration. The hyperactivity syndrome, representing metabolic dysfunction (e.g., uncouplers of oxidative phosphorylation), is characterized by greatly accelerated locomotor activity, overreaction to stimuli and increased ventilatory activity. The physical deformity syndrome, indicative of neurological dysfunction (e.g., AChE inhibition), is characterized by a high incidence of convulsions, tetany, scoliosis/lordosis and hemorrhage in the vertebral area. This classification approach also should be useful to help delineate specific, underlying mechanisms or sites of chemical action.