Changes in gene expression which occur during periods of environmentally induced stress provide models for the study of gene regulation. Several types of stress have been shown to elicit a specific and reproducible pattern of gene expression in various plant species. These stress factors include heat shock, anaerobiosis, plant pathogens, oxygen free radicals, heavy metals, water stress, and chilling. In some cases, changes in specific genes have been identified, such as increases in the expression of the gene encoding the phytoalexin-synthesizing enzyme in pathogen elicitor-treated cells. However, in most cases, the functional identity of stress-induced genes is unknown. The alterations in gene expression during stress usually are rapid and repeatable, making these genetic systems ideal for examination of factors and mechanisms involved in gene regulation.