This paper estimates the relationships between bidding quantities, marginal cost and market power measures in the Spanish wholesale electricity market for two different regulatory periods: 2002-2005 and 2006-2007. Using panel econometric techniques we find differences in the impacts on bidding strategies for both periods. Hence, the marginal cost and the market power measures affect bid and net quantities. The market power measures also suggest that the coefficient is consistently positive and highly significant for both periods. Moreover, the market power and marginal costs have mixed effects according to the models proposed for both periods. In addition, our results point to the effectiveness of the different effects of mitigating the market power in the Spanish electricity market. For the 2006-2007 period, the proposed causal relationships are partially validated by the cointegration results, which assumes there is a significant causality between the Lerner Index and the marginal cost. (C) 2014 Published by Elsevier Ltd.