AbstractA correlation approach was developed to describe narcosis in several aquatic species. By assuming that narcosis occurs as a result of a constant volume fraction of toxicant at the target site in the test organism, it is possible to correlate LC50s for a series of chemicals with their octanol‐water partition coefficients and molar volumes. The approach was successfully tested on acute and chronic toxicity data from the published literature. Two hundred‐fourteen toxicity determinations, with 113 different organic chemicals and involving fathead minnow (Pimephales promelas), guppy (Poecilia reticulata), golden orfe (Leuciscus idus melanotus), water flea (Daphnia magna), brine shrimp (Anemiaspp.) and Paramecium (Tetrahymena elliotti), were employed.The data analysis suggests that partition coefficients of the chemicals between the target phase, i.e., the site of toxicant action, in the test organism and water are similar to, but generally lower than, octanol‐water partition coefficients. The analysis also suggests a method of correlating the toxicities of mixtures and provides an explanation of the “toxicity cut‐off” frequently observed in toxicity tests with chemicals of low water solubility. The interpretation provides insights into the role of partitioning in the acute and chronic effects of nonselective narcoti
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