Theoretical quantitative structure-activity relationships have been established based on narcotic mechanisms of action and toxicity data to the fathead minnow, Daphnia magna and Vibrio ®scheri. The results con®rmed that hydrophobicity is important in modelling narcosis. However, the results of the present investigation also con®rmed that molecular bulk may play an important role in toxicity to aqueous organisms, especially in the toxicity to organisms with lower lipid content, for example, V. ®scheri. The results are discussed in the light of other inter-species differences that may occur as a consequence of accumulation and clearance kinetics and the relative metabolism of xenobiotics by each organism. Because of the complex tissue structure of organisms, classi®cation of narcotic compounds using hydrophobicity parameters alone can sometimes lead to errors.Abbreviations: QSAR, quantitative structure-activity relationship; P, 1-octanolawater partition coef®cient; Vx, McGowan characteristic volume; Vi, Leahy's intrinsic volume; V M , molar volume; BCF, bioconcentration factor; EC 50 , concentration causing 50% inhibition of bioluminescence to Vibrio ®scheri; IC 50 , immobilisation concentration for 50% of Daphnia magna at 24 hours; LC 50 , lethal concentration for 50% of fathead minnow at 96 hours.