The Brazilian retail sector has gone through expressive structural transformations since the 90’s decade. Despite the high level of concentration, the retail sector has presented evidences of strong rivalry among the top firms. This research aimed to analize the impact of market concentration on the use of market power for specific agricultural products, through time series analysis of agricultural prices referring to the São Paulo state. The empirical results indicated that the use of market power differs from product to product, what suggests that retailers have been adopting specific strategies according to the characteristics of products demand. The analysis of price asymmetry showed that retailers tend to delay the transmission of price decreases and transmit price increases more rapidly. Moreover, results indicate that the increase in rivalry in the current decade has made it more difficult for retailers to use their market power manipulating prices. In general, retailers were able to transmit price increases more intensely before the year 2000, when the turnover level was lower and restricted to the small and middle size firms.