The Grošnica Reservoir is the oldest water-supply reservoir in Serbia. Investigations carried out monthly during the two-year period of 1996-98 revealed certain changes of some chemical and biological characteristics in relation to investigations performed almost 50 years earlier. Changes were noticed in composition of the plankton community and in some physical and chemical parameters. Although it could be expected that eutrophication would occur in time, the results do not confirm it. Total P, Secchi disk, and chlorophyll α values indicate that the Grošnica Reservoir belongs to the category of mesotrophic waters. The noticed differences can be attributed to raising of the dam's height and increasing the lake’s volume, resulting in reduction of the influence of turbid currents, as well as to forestation of the reservoir's banks.