Gibberellic acid (GA(3)) is reported to have diverse effects on hairy root cultures of many plant species; therefore, the effects of GA(3) on the growth, secondary metabolite production (caffeic acid derivatives and lignin), phenylalanine ammonia lyase (PAL) activity, and free radical scavenging activity of light-grown Echinacea purpurea L. hairy roots were investigated. Eight concentrations of GA(3), ranging from 0.005 to 1.0 mu M, were added to shake flask cultures. The moderate GA(3) concentration, 0.025 mu M, resulted in the highest concentrations of cichoric acid, caftaric acid, and chlorogenic acid, as well as increased PAL activity, cell viability, and free radical scavenging activity, while higher and lower GA(3) concentrations resulted in reduced levels compared to the control (lacking GA(3)). The moderate GA(3) concentration also affected root morphogenesis; supplementation with 0.025 mu M GA(3) resulted in the development of thick, dense, purple-colored roots, while roots exposed to the higher and lower concentrations of GA(3) were thin and off-white. This study demonstrates that supplementation with GA(3) may be an excellent strategy to optimize the production of secondary metabolites from E. purpurea hairy root cultures; however, the GA(3) concentration is a critical factor.