According to the results obtained in this study, drought stress can enhance the accumulation of silymarin in milk thistle seeds. Moreover, under drought stress, the share of silybin increased which possess the greatest degree of biological activity among the silymarin components. Silymarin, an isomeric mixture of flavonolignans found in milk thistle (Silybum marianum (L.) Gaertn) seeds, has been used for its hepatoprotective effects for more than 2,000 years. Biosynthesis and accumulation of active substances like silymarin in plant tissues highly interacts with the environmental conditions. Effects of moderate and severe drought stress (based on soil moisture depletion) on silymarin content and composition in milk thistle seeds were evaluated in a field study. Averaged across treatments, milk thistle seeds contained 19.3 g kg(-1) silymarin. Drought stress enhanced silymarin accumulation in milk thistle seeds. Plants grown under moderate and severe drought stress treatments contained 4 and 17 % greater silymarin than those grown in well-watered condition, respectively. Greater content of sylimarin in stressed plants was attributed to more contents of silybin, isosilybin and silychristin, while silydianin content was lower under drought condition. According to the results obtained in this study, drought stress enhanced accumulation of silymarin in milk thistle seeds and improved its quality by increasing the share of silybin, which possess the greatest degree of biological activity among the silymarin components.