Mentha x piperita. L herb was treated with O-3 at 22 degrees C and relative air humidity 60-70% for 90 min and subjected steam distn. to recover the essential oil. The oil was studied by gas chromatog. for chem. compn. and treated with 1,1-dipheny1-2-picrylhydrazyl radical to det. the radical scavenging activity of the oil. Terpenes, flavonoids, phenolic acids and polyphenols were main components of the oil. The ozonation resulted in decreasing both the oil yield and contents of individual oil components (except for short exposition to 03 for less than 15 min). The content of metabolities in the oil showed a max. after 60 min long ozonation. The antioxidn. activity of the oil increased slightly after ozonation.