Sometimes, metabolites and degradation products are cause of toxicity. QSAR Toolbox provides two types of metabolism – observed and simulated. All generated metabolites (documented and simulated) could be searched and profiled by the same approach as the parent chemicals and thus could inform the user about the potential formation of toxic metabolites. Additional information concerning the studied species and experimental conditions is provided for the observed metabolisms. The metabolic simulators imitating biotic (microbial, liver and skin metabolism), as well as abiotic (autoxidation, hydrolysis) transformations. Propagation of the simulated metabolism is confined objectively using parameters associated with the formal rate constants of metabolic transformations. Generation of metabolites is additionally reduced limiting the levels of metabolism; e.g. three levels of metabolism were found adequate to simulate rat liver S9 metabolism.