A discussion of the tidal, wind-driven, and density-driven aspects of the circulation in Bay of Ilha Grande and Bay of Sepetiba (Brazil) based on historical data and hydrographie and current mete'r data collected in September of 1977, is presented in this paper. The data analysis has shewn a remarkable contrast between the circulations of these two adjacent Bays. The circulation in Bay of Sepetiba is strongly tidalwhereas Bay of Ilha Grande has a weak tidal circulation superimposed by a quasi-steady flow induced by density gradients. The stratification is maintained because Bay of Ilha Grande, beeing deeper, allows colder and saltier shelf water to flow eastward towards the fresher and warmer water flowing out of Bay of Sepetiba, highly influencedby river runoff. The velocity and density fields have shown seiche oscillations that agree very well with theoretically derived modes inherent to the local geometry and density stratification.