Aim: The aim of this study was to analyse the expression at the mRNA level of cytochromeb, subunits COI, COII, COIII of complex IV, ATP6 and ATP8 and to perform aquantitative analysis of the activity of ATP synthase as complex V of the respiratory chain, responsible for ATP synthesis and break down of ATP to ADP in breast cancer. Methods and Results: The level of gene expression at the mRNA level was evaluated on the basis of the light intensity of fluorochrome or fluorescein in the cells. Determination of the activity and location of the ATP synthase in cells was carried out using hybridization. In the formulations of breast cancer there was a higher genes expression at the mRNA level for all the examined genes as compared with non-cancerous tissue. The ATP activity was also higher in preparations obtained from breast cancer cells compared with the control tissue. Conclusions: The results confirm mtDNA incorporation to nDNA in neoplastic cells. They point to increased expression at mRNA level for COI, COII, COIII, ATP6 and ATP8 in breast cancer cells compared with control tissue. Increased amount of ATP synthase points to increased ATP in a neoplastic cell