Abstract The aim of this paper was to investigate baseline toxicity to rats and effect of exposure routes on toxicity in rats and fish. In this paper, 1588 industrial chemicals were selected to investigate baseline toxicity to rats. The results showed that rat toxicity varies around a constant for classified compounds or homologues. The toxic contributions of substituted functional groups have been calculated and alkanes were used as baseline toxicity. The toxic contributions, equal to toxic ratios (TR), show that small changes in chemical structure can result in different toxic effect in rat toxicity. However, this situation has not been observed in fish toxicity because the threshold of excess toxicity (e.g. log TR=1) was too high to distinguish differences in toxicity. Very close critical body residues (CBRs) calculated from percentage of absorption and bioconcentration factors indicate that most of aliphatic chemicals may share the same modes of toxic action between rat and fish species. The high estimation error of bioconcentration factor calculated from computer programs for some compounds suggests that classification of excess toxicity should be based on the CBRs, rather than the TR because the TR is closely related to the exposure routes.