Among the organisms used as biomonitors, oysters stand out due to the large amounts of contaminants that accumulate in their tissues. The objective of this research was to determine the physicochemical parameters of the estuarine system at three points in São José Bay, Maranhão - Brazil, to perform the biometry of oysters, and to quantify nitrites (NO2-) in the intravalvular fluid of oysters. The physicochemical parameters of the brackish water showed that dissolved oxygen presented values below the levels recommended by Brazilian legislation. The pH and temperature remained within the established limits. In the biometric analysis, it was possible to observe that the oysters from harvesting areas are smaller in all dimensions, especially during the rainy season. The result of the quantification of NO2- in the oyster supernatant revealed significantly higher concentrations in the harvesting area, showing that this area is the most impacted when compared to the urban area and farm area (control group). The results indicate that the quantification of NO2⁻ by the modified Griess reaction is a valid, simple, effective and low-cost method that can be used in environmental monitoring, in addition to the physicochemical parameters of the water and oyster biometric analysis. The use of oyster supernatant for NO2- quantification proved to be a new sampling unit that is easy to collect. The present study may help in biomonitoring studies through the quantification of NO2- in the oysters’ supernatant.