This report describes the results of a preliminary study relating to increasing the power output of hydroelectric power plants at existing reservoirs in Southern Norway subject to the constraints of current regulations regarding highest and lowest regulated water levels (HRWL and LRWL). The main scenario involves twelve new power stations with a combined power output of 11,200 MW. It is envisaged that these power stations would be constructed with new tunnels to an upstream reservoir and to the downstream outflow into a reservoir, a fjord or the sea. Five of the power plants are pumped storage power stations with a combined output of 5,200 MW, while the remainder are hydro storage hydroelectric power stations with a combined output of 6,000 MW, all but one of which discharge into a fjord or the sea. None of the selected power stations will experience more rapid water level variations than 14 cm per hour in the affected upstream and downstream reservoirs. The strictest restrictions on water level variation are associated with downstream reservoirs. In most cases it will take 2-4 weeks of constant power generation to empty an upstream reservoir. It is assumed that the operation of the existing power station will remain unchanged. The most serious environmental challenges affecting reservoirs resulting from increased power generation installation are connected with the risk of increased erosion, changes in circulation, changes in water temperature, reduced ice cover and increased danger of unstable ice. All these physical changes can have an impact on ecosystems. Many of the selected reservoirs are already strongly affected by water level regulation. In reservoirs which receive pumped water from lower reservoirs or neighbouring water systems, environmental impacts can be considerable because transferred water can result in major changes in water chemistry and temperature, and a range of organisms may be transferred from the lower reservoir to the upper one. Knowledge of the possible effects of such water transfer is incomplete. The environmental challenges connected with balance power will vary from project to project and will depend on the type of operational pattern and restrictions implemented. Our ability to develop and use knowledge of environmental effects will determine what sort of local impacts balance power projects may have. Each of the power station installations studied will require connection through a separate 420 kV line to appropriate points in the central supply grid if power exchange with other countries is to take place by way of the central transmission grid.