The toxicity of menthol was studied on 4 different in vitro systems covering organ, cellular and subcellular levels. The 50% inhibitory concentration (IC50) for the cellular and subcellular systems ranged from 0.32 mM to 0.76 mM. At a concentration of 0.5 mM menthol the receptor mediated respiratory stimulation of isolated brown adipocytes was markedly inhibited while the intracellular mitochondrial functions were still unaffected. However, using isolated rat liver mitochondria 0.5 mM menthol was found to cause increase in the 'state 4' respiratory rate and osmotic swelling, indicating a leakage of the mitochondrial membrane. We therefore suggest that one effect of menthol is a deterioration of biological membranes. For the determination of the cellular toxicity of foreign compounds isolated brown adipocytes represents a convenient and sensitive model, providing the possibility to localize the primary site of action in terms of mitochondrial or extramitochondrial level.