In order to stimulate the quality of the service, ANEEL introduced penalties in the contracts with the concessionaires of public services of transmission of electric energy in the case which the transmission facilities are turned off, be for accident or for maintenance. In this sense, a variable value was instituted whose amount is discounted of the transmission revenue. The value of this parcel is not of easy determination, and ANEEL, based on observations, established a formula for calculation. High penalties can result in bad incentives to the entry of companies in the sector; otherwise, low penalties can induce to the offer of service of bad quality. By using the Theory Principal-agent, this work search to determine this formula so that the true objective is reached. in a rational way. In the present work we analyze the relationship ANEEL - Concessionaires by using the Theory Principalagent. The main conclusion of the work, besides the rising of the costs and of the analysis of the behavior of the concessionaires, refers to the fact that the values of the parameters used by the regulator can be established in smaller values than the now adopted, with larger economic efficiency.