Informal economy of gold mining has contaminated some important river basins in Amazon.

Follow-up studies on critical compartments showed some areas with high Hg levels in fish as well as in human hair samples.

Average Hg in piscivorous fish in the Madeira river itself was 846 ppb (N=284) with a maximum of 3921 ppb.

Mercury in fish from non polluted areas in this basin shows high variability, even for single species.

A seasonal variation in Hg content was observed, with higher values at the end of the dry season.

In the upper Tapajos basin comparable values were found for fish but with a definite decreasing trend downstream.

Average value for piscivorous fish in the whole Tapajos basin is 482 ppb (N=122) with a maximum value of 3770 ppb.

Hair Hg was higher in fishing villages in the Tapajos (average : 17 ppm ; with N=432 and maximum value of 176 ppm) than in the Madeira (average : 9 ppm ; N=169 ; maximum 71 ppm), and data from some areas of the Tapajos suggest a decrease with time.

Mercury was much higher in urine of goldshop workers in Santarém (low Tapajos) than in Alta Floresta (high Tapajos) and show a decreasing trend in both cases, probably related to the significant decline in gold mining activities during the study period (1986-1994).