The physiological responses of rainbow trout to acutely lethal aqueous concentrations of the suspected polar narcotics phenol, 2,4-dimethylphenol, aniline, 2-chloroaniline and 4-chloroaniline were examined. Visible signs of intoxication included tremors that progressed to whole-organism clonic seizures, followed by general depression and respiratory-cardiovascular collapse. Tremors and seizures were usually initiated with coughs. The most striking changes in the respiratory-cardiovascular parameters for all five toxicants included elevated cough frequency (coincident with seizures), ventilation frequency and hematocrit, and depressed gill oxygen uptake efficiency, arterial pH, total arterial oxygen and total arterial carbon dioxide. The physiological responses noted were attributed to the initial muscular activity associated with seizures followed by respiratory-cardiovascular collapse. Using phenol as a model compound, these effects were found to be reversible when intoxicated fish were exposed to chemical-free water. Principal components analyses of the physiological responses demonstrated that the 11 monitored variables were highly correlated. Interpretation of the principal components showed that responses within each toxicant-exposure group were distinct from those of the control group. The response data sets for the suspected polar narcotics (N = 20 fish) were combined with four fish acute toxicity syndromes (FATS) (N = 32 fish) previously described (respiratory uncoupler syndrome, respiratory irritant syndrome, acetylcholinesterase inhibitor syndrome and narcosis syndrome), and assessed using discriminant function analysis. This analysis showed that all 52 trout could be correctly classified into their respective FATS. As a result, two separate narcosis FATS, narcosis type I (previous work) and narcosis type II (this study), were defined. Identification of a narcosis type II syndrome supports the concept of developing quantitative structure-activity relationship models for polar narcotics in fish.