Monitoring of tropospheric ozone was realized in the chosen forest ecosystem of the Western Carpathians Mountains. The studies are carried out at 5 monitoring sites – Malé Karpaty Mts (Geldek), Malá Fatra Mts (©tefanová), Kozie chrbty Mts (Východná), Javorníky Mts (Turkov) and Turzovská vrchovina Mts (Riečky). The ozone measurement took place during vegetation period by two ways – by passive samplers (filters changed two times per month) and by Thermo Environmental Model 49 Ozone Analyzer installed at the locality of Východná. Measurements were carried out in the period of April–October in 1997–1999.