The majority of industrial organic chemicals lack identifiable structural characteristics that result in specific biological activity. These nonpolar-nonelectrolytes are acutely toxic to aquatic organisms via a nonspecific mode of action termed narcosis. The toxicity of industrial chemicals eliciting nonpolar narcosis can be reliably predicted by log P (baseline toxicity models). Using single chemical and joint toxic action models, several research groups have reported classes of polar compounds (for example, esters, phenols, and anilines) that elicit a narcosis-like syndrome; however, they are more acutely toxic than what is predicted using baseline toxicity models. An assessment of rainbow trout (Salmo gairdneri) in vivo respiratory-cardiovascular responses during intoxication by polar narcotic phenol and aniline derivatives established a toxicity syndrome unique to that elicited by nonpolar narcotics. This finding further suggests a mode of action unique to polar narcotics and supports the use of structure-activity relationships specific for these compounds. The proposition that there may be multiple mechanisms or sites of narcotic action is compatible with recent studies concerning the cellular and molecular mechanisms of anesthetic action.