The article presents data on the content of biologically active compounds and the total activity of phenolic antioxidants in the leaves and inflorescences of the valuable forage plant Trifolium pannonicum Premier cultivar grown in different agroclimatic subareas of the Novosibirsk region – moderately warm, moist and moderately warm, not sufficiently moist. It is shown that, depending on the plant growth, biologically active substances are synthesized in differently: the content of flavonols, catechins, carotenoids and pectin substances is 1.1–1.6 times higher in plants from a more humid area. Tannins maximally (up to 17.43%) accumulate in the leaves of T. pannonicum, grown in an area with insufficient moisture. All samples of T. pannonicum variety Premier are characterized by high antioxidant activity of ethanol extracts from leaves and inflorescences (1.50–2.20 mg/g). The maximum of the total activity of antioxidants was found in the inflorescences of T. pannonicum from an area with insufficient moisture. Plant material T. pannonicum variety Premier is characterized by a fairly high content of biologically active substances.