Oceanographic and isotopic investigations in the Caspian Sea and the analyses of the available data on the discharge to the sea and the observed sea level changes suggest that climatically caused changes of river inflow are the major cause of the sea level fluctuations over the last century. Hydrogen-3 and 3H-3He data indicate that the deep basins of the sea are rapidly ventilated, although the hydraulic turnover time of the sea is approximately 200 years. The concentration levels of the anthropogenic radionuclides 90Sr, 137Cs and 239,240Pu in the water column can be explained by global fallout and therefore, at the sampling sites visited, there were no signs of dumping of radioactive wastes. The anthropogenic radionuclide data support the idea of fast exchange of water masses in the Caspian Sea. The isotopic and oceanographic data collected during the cruises have shown potential to allow for a better understanding of the water circulation in the Caspian Sea.