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. The finding further suggests a mode of action unique to polar narcotics and supports the use of structure-activity relationships specific for these compounds. (Copyright (c) American Society for Testing and Materials.)