The analysis of photosynthetic activity ofHelianthus tuberosusleaves was based on thefield experiment carried out in 2003-2005 in The Experimental Station in Parczew on good ryecomplex soil. The experiment was performed by means of randomized sub-blocks in 3 replications,including factors: the 1st order - varieties(Albik and Rubik), and the 2nd order – mineral fertilizationat rates of: N₀P₀K₀, N₀P₄₄K₁₂₅; N₅₀P₄₄K₁₂₅; N₁₀₀P₄₄K₁₂₅; N₁₅₀P₄₄K₁₂₅; N₂₀₀P₄₄K₁₂₅, recalculated ontothe elemental forms on a background of full rate of FYM (30 t ha⁻¹). The plant’s photosyntheticactivity during flowering was evaluated by means of leaf chlorophyll fluorescence inductionmeasurements applyingPAM-2000 device. Following fluorescence parameters were determined:maximum efficiency of a photosystem PS II (FV/Fm), current number of electrons transported in PS IIunder conditions of light adaptationФPSII; efficiency of open units PS II – FV’/Fm’; coefficient ofphotochemical (qP) and non-photochemical fluorescence quenching (qN). It was found thatmaximum efficiency of the photosystem PS II (Fv/Fm) was the most stable chlorophyll fluorescenceindicator forHelianthus tuberosus, while coefficient of non-photochemical fluorescence quenching(qN) – the most changeable. Rubik cv. appeared to be the variety having higher values of Fv/Fm,ФPSII,qP, and qN, as well as their better stability, than Albik cv. Mineral fertilization applied atHelianthustuberosuscultivation contributed to the decrease of primary photosynthetic reactions in photosystemII (Fv/Fm) and disturbances of the efficiency of open units PS II (Fv’/Fm’). The decrease ofchlorophyll quantum efficiency (ФPSII), photochemical chlorophyll fluorescence quenching, andnon-photochemical chlorophyll fluorescence quenching (qN) were also observed.