Oxidative stress-induced conditions mobilize cell reserves in order to mitigate the harmful effects of excessive free radicals production and to stop the processes of lipid peroxidation. The ability of cells to decrease the intensity of lipid peroxidation reactions can be assessed by determining the iron reducing power. In order to monitor the adaptive capacity of spirulina, grown in the presence of Zn(II) acetate and Fe(III)-alanine as stimulators of protein synthesis, cyanobacterium has been subjected to oxidative stress induced by fluctuating light regime. Throughout the adaptation period, Spirulina platensis was grown in the presence of 15 mg/l Zn(II) acetate and 50 mg/l Fe(III) alaninate under optimal conditions with continuous illumination. On day 3 of cultivation cycle, corresponding to the early exponential growth phase, the culture has been exposed to light-induced stress by reducing light period within 4 hours of 24 hours. This regime has been followed until day 7 of cultivation cycle. It was used as control spirulina, grown in the absence or presence of chemical stimulators under optimal conditions. In algal biomass, collected at intervals of 24 hours, it was determined the iron reducing power, expressed as mg/ml ascorbic acid equivalents. Under conditions of continuous illumination, tested compounds increased significantly the capacity of biomass to reduce iron ions. In experimental variants applying Zn(II) acetate, the value of iron reducing power in early exponential growth phase was 36% higher compared to control samples. Elevated levels were maintained throughout cultivation cycle and in early stationary phase were 30-39% higher. Fe(III) alaninate from cultivation medium led to an increase of iron reducing power of 1.4 times at the beginning of exponential growth. On day 5 of cultivation, iron reducing power was about 65% higher compared to control. In stationary phase, iron reducing power was about 87% higher compared with control samples. Under conditions of light-induced stress reducing to 4 hours of light period, the presence of chemical stimulators in spirulina cultivation medium decreased the values of iron reducing power. Zn(II) acetate decreased iron reducing power with 62% on day 5 of spirulina cultivation under light-induced stress, culture being in exponential growth phase. In the following days, iron reducing power of biomass was maintained at values below control samples. In the presence of Fe(III) alaninate, on day 5, iron reducing power decreased by 53% and remained low during the exponential phase. Reducing this parameter under light-induced stress revealed the involvement of supplemented compounds in biosynthetic processes of spirulina that depleted the antioxidant capacity of culture. Therefore, the iron reducing power assay in spirulina biomass can confirm induced oxidative stress.