Aspects of continuing nuclear power plant operation in Lithuania are discussed in the paper. The major factors influencing the development of energy sector over a long-time period are analysed: growth of electricity demand, future scenarios of fuel price development, expected changes in electricity production cost in electricity markets of neighbouring countries, construction of new generating capacities using fossil fuels and nuclear energy, energy security. The method of the performed analysis is based oil application of all econometric model including uncertainty analysis (for electricity demand forecasting) and the MESSAGE model (for optimization of the development of energy supply systems, including options of utilisation of energy resources, modernization of existing energy technologies and construction of new power plants, implementation of environment protection measures, etc). The most probable path of electricity demand growth, including its feasible fluctuation in a range of +/- 10%, within the period 2000-2025 is presented. It is shown in the paper that in the case of construction of comparatively small power plants, Lithuania will have enough capacities to meet the growing demand until 2014. Factors influencing competitiveness of new nuclear power are analyzed, principles of energy security are discussed.