The aim of the study was to determine the effect of blue (450 nm) and red (630 nm) LED irradiation with different energy exposure on the activity of mitochondrial cytochrome c oxidase. Methods. The object of the study was mitochondria isolated from the liver of the Japanese quail Coturnix japonica. Cytochrome c oxidase activity was measured by the oxidation rate of reduced tetramethyl-p-phenylenediamine in the presence of rotenone. Results. It was shown that irradiation of mitochondria with blue light with energy exposures not exceeding 4 J/cm² causes approximately 5-15% stimulation of cytochrome c oxidase activity, and doses greater than 5 J/cm² lead to inhibition of the enzyme activity. Irradiation of mitochondria with red light also has a stimulating effect (10-20%) on cytochrome c oxidase activity compared to non-irradiated samples. Conclusion. It is suggested that low-dose blue light irradiation may have similar therapeutic effects to red light in photobiomodulation.