Paraoxonase 1 (PON1) is a member of a three-gene family (PON1, PON2, and PON3). PON1 activity dominates in human plasma. It is secreted from hepatic cells and is found in the circulation bound to high-density lipoproteins (HDLs). For many years it has been known only for its ability to hydrolyze organophosphate derivatives. More recently, PON1's antioxidant activity draws attention as the enzyme was described to prevent oxidation of lipoproteins by reactive oxygen species formed during oxidative stress. PON1 was also shown to hydrolyze atherogenic products of oxidative lipid modification such as phospholipid peroxides and cholesterol ester hydroperoxides. Some studies indicate that the enzyme presents a lipolactonase activity and hydrolyzes homocysteine thiolactone (HCTL). There is growing evidence as to PON1's protective role in atherosclerosis. Genetic (PON1 polymorphism) and environmental factors and lifestyle may influence PON1 blood concentration and biological activity. Among the many recognized factors accounting for lifestyle, physical activity plays an important role. Various, often opposite, effects on PON1 status are observed in regular training and single physical activities. The results of different studies are often contradictory. It may depend on the time, intensity, and frequency of physical activity. Additionally, it seems that the effects of physical activity on PON1 blood concentration and activity are modified by environmental and lifestyle factors as well as PON1 polymorphism.