The effects of thermal injury (cold and heat shock) and ozone treatment on lipoxygenases 1 (LOX-1) and 2 (LOX-2) of soybean seedlings have been investigated. Cold stress led to a decrease of the specific activities of both isoenzymes, attributable at least in part to a down-regulation of gene expression at the translational level. Both heat shock and ozone treatment enhanced lipoxygenases-specific activities, acting at the level of transcription of the genes. It is proposed that LOX-1 and LOX-2 are involved in the thermotolerance of soybeans and in the precocious aging induced by ozone.