One-hundred fish samples of different species, capture between October and Novenber 1989 from Paraguay and Paraná rivers, were evaluated in terms of total mercury content in fresh meat. Specimens were captured at Bahía Negra, Valle-mí, Concepción, Asunción and Pilar on Paraguay river, and at Ayolas an Paraná river. The samples captured at different locations on Paraguay river showed a mercury content of 250.3mg/kg. Consindering the meam values for mercury content in the fishes from Paraguay river from north to south, the lowest value was recoded in Bahía Negra. The mean values of the samples from the other locations on Paraguay river ranged from 209.5 mg/kg on Asunción, to 307.2 mg/kg on Pilar. The mean mercury concentration for the samples from each location are below the safety limit (500 mg/kg), but some specimens showed higher values. It is very interesting to note that the specimens procured from Paraná river (Ayolas) showed the higher mean value for the mecury content. There is certain pattern in the species showing both the lowest and the highest mercury concentration vary in each location. This fact cuold be related more with the food intake charasteristcs of each species more than with the location of the capture. We found that specimens of Piaractus mesopotamicus (pacú) and Doradidae sp. (armado) from different places showed the lowest mercury contents, and specimens of Pseudoplatystoma coruscans (surubí) and Salminus brasiliensis (dorado) showed the highest levels