Twenty flavonoids isolated from plants or transformed into methyl or acetyl derivatives were tested with regard to their influence on cyclooxygenase from the ram seminal vesicle microsomes and lipoxygenase from soya beans. Moreover, their antioxidant properties were evaluated by estimating the amount of the malonylaldehyde formed from arachidonic acid. Only rhamnetin and myricetin inhibited the soybean lipoxygenase. Most of the tested flavonoids stimulated cyclooxygenase at a high (100 microM) substrate concentration, myricetin being the most potent. Rhamnetin was the strongest antioxidant, while myricetin was about ten times weaker. Structural requirements for the cyclooxygenase stimulation, lipoxygenase inhibition and antioxidant properties were different in the case of the twenty tested flavonoids.