To evaluate the effects of radioactivity on active plant substances, samples of fennel (Foeniculum vulgare Miller., fruit), guarana (Paullinia cupana, Kunth, seed), ginkgo (Ginkgo biloba, L., leaf), and kava-kava (Piper methysticum G. Forst, rhizome), were treated with scaled doses (0-25 KGy) of gamma radiation (Co60) using the "blind test" methodol.The active substances from each sample (fennel essential oil, guarana caffeine, ginkgo flavonoids, kava-kava terpene lactones) were analyzed by qual. and quant. methods after irradiationNo significant differences were seen between the control sample (0 KGy) and the irradiated samples with regard to the mol. structures of the active substances.Microbe contamination, however, was significantly reduced, about 10,000 CFU/g, with the initial 5 KGy dose.Thus, gamma radiation can be used as an alternative procedure to reduce microbiol. contamination in medicinal plants.Before this procedure can be extended to other medicinal plants, more specific anal. methods are recommended to verify possible structural alterations in active vegetal mols.