The dynamics of urea and dissolved free amino acids (DFAA) were studied during phytoplankton bloom in six outdoor experimental ponds for 40 days from August to September 1992. Sand filtered lake water was continuously flowed into the ponds, which have regular hexagonal shape with 1.75 m in depth and 36 m³ in volume and were adjusted to water renewal rate of 20 days. Cyanobacterial bloom occurred in all ponds during the first 20 days and after that green algal bloom followed.<BR>The changes in urea concentrations were characterized by the appearance of at least two peaks. The maximum urea concentrations ranged from 4.47 to 7.93μmol⋅1^-1. The peak of urea concentrations coincided with high zooplankton biomass or density (the dominant species were Brachionus calyciflorus and Moina micrura). Zooplankton density increases followed rapid declines of phytoplankton biomass. These results suggested that the high concentrations of urea resulted from excretion by zooplankton grazing on phytoplankton. The relationships between increases in urea concentration and zooplankton biomasses were highly significant (r=0.71, p <0.001 for the first peaks, r=0.60, p<0.01 for the second peaks). The specific excretion rates of urea by zooplankton were estimated to range from 0.07 to 0.30 μmol⋅mg dry wt^-1⋅d^-1.<BR>This study also showed an increase of DFAA during periods of high zooplankton biomass. The maximum DFAA concentrations ranged from 3.73 to 8.26 μmol⋅1^-1. Zooplankton excretion and grazing activity were also suggested to be the important sources of DFAA to the water. During periods of high zooplankton biomass, DON ranged from 0.41 to 1.09 mg N⋅l^-1 and made up between 7 and 54% of TDN. Urea and DFAA occupied 3-27% and 4-13% of DON, respectively.<BR>The present study suggested that zooplankton grazing activity can be an important source of urea and DFAA in freshwater environment.