In perfectly competitive markets, there are no loopholes that can be exploited by agents to maximize profit through non-competitive behavior. However, electricity markets look more like an oligopoly than perfectly competitive markets. If competition is not perfect, generation agents may try to establish optimum price bidding strategies to maximize profits. If an agent is able to increase profits through other means than reducing costs, it is said to have market power. This thesis investigates the possibility of establishing optimum price bidding strategies in the short- term market of systems with hydroelectric predominance, with emphasis in the Brazilian system. The possibility of establishing optimum price bidding strategies and thus market power are evaluated in the two possible price formation schemes used in hydrodominated systems. The one currently used in Brazil, known as tight pool, where price are determined centrally through optimization models, and where only the thermal generators bid prices. Market power is also evaluated in a general price bidding scheme, where hydro generators also bid prices. Market power is evaluated trough an oligopoly model from game theory and through market concentration measures. In establishing optimum price bidding strategies, one crucial issue is modeling competitors`s price bidding behavior. Due to the lack of historical data, the price bidding behavior of the competitors is modeled by triangular probability distributions, equivalent to some membership functions typically used in fuzzy modeling. It is shown for the Brazilian system that in a tight pool scheme the market power and thus the possibility of establishing optimum bidding strategies are very limited. On the other hand, in a general price bidding scheme the market power of hydro generators may be significant and aggressive behavior may lead the system to critical supply conditions, if no market power mitigation mechanism is implemented.