Phytoalexin production is one of a number of inducible plant defense reactions which is thought to confer disease resistance against microbial infections in plants. Soybean (Glycine max) tissues produce and accumulate isof lavonoid phytoalexins (glyceollins) following either inoculation with a soybean pathogen, the fungus Phytophthora megasperma f.sp. glycinea, or treatment with a β-glucan elicitor isolated from the fungal cell walls. Studies on the regulation of phytoalexin biosynthetic enzymes suggested that the phytoalexin defense response in soybean is controlled by temporary gene activation. The activities of several of the glyceollin biosynthetic enzymes have been studied during race-cultivar specific interactions between P. megasperma zoospores and intact soybean primary roots, a natural site of attack by the fungus. Only in the incompatible (host-resistant) interaction, is there an early enhancement of the enzyme activities starting at 2–4 h after inoculation, which correlates with the onset of glyceollin accumulation and expression of the hypersensitive response. Elicitor treatment of soybean cell suspension cultures causes major changes in the population of total translatable mRNA, which indicates large metabolic changes associated with phytoalexin synthesis and possibly other defense responses of the challenged cells.