The acute toxicity of aliphatic and aromatic mono and diesters in two eucaryotic organisms was compared. The test systems were the static 2-d Tetrahymena pyriformis 50% population growth impairment (IGC50(-1)) assay, and the flow-through 4-d Pimephales promelas 50% mortality (LC50(-1)) assay. In ciliates, esters act via the nonpolar narcosis mechanism of toxic action. This was indicated by: the high quality 1-octanol/water partition coefficient (log Kow) dependent quantitative structure-activity relationship (QSAR), log IGC50(-1) = 0.79 (log Kow) - 1.93, n = 15, r2 = 0.945, s = 0.22, f = 222.37 Pr > f = 0.001); volume fraction (Vf) (0.8e-02); and "a" coefficient (0.3) which are not different from other nonpolar narcotics. In vivo hydrolysis in Tetrahymena appears to be insignificant. However, in fish, presumably because of more active esterases, in vivo hydrolysis is significant and leads to greater toxicity of esters than observed for nonpolar narcotics. Moreover, it leads to a unique high quality QSAR, log LC50(-1) = 0.64 (log Kow) - 0.64, n = 14, r2 = 0.945, s = 0.22, f = 207.08, Pr > f = 0.0001). Due to in vivo hydrolysis, a nonreducing concentration gradient is formed between water and fish. Therefore, the fish take up more toxicant as compared to a situation that leads to thermodynamic equilibrium. Additional information about the mechanism of ester toxicity in fish was gained by applying corrections for hydrolysis in volume fraction analyses. The corrected Vf (0.6e-02) is very close to the one found for nonpolar narcotics (0.7e-02).(ABSTRACT TRUNCATED AT 250 WORDS)