The potential for release of ammonium (NH4+) and/or urea from the pool of dissolved organic nitrogen (DON) was examined in samples taken from Lake Kinneret (Israel), the River Charente estuary and coastal water near lie de Ri! (French Atlantic coast). After prefiltration through 1.0 or 1.2 mu m membranes to remove most of the microbiota with the exception of bacteria, water samples with or without supplements (40 mu M) of various organic nitrogen compounds (arginine, glucosamine, guanine, hypoxanthine, lysine, ornithine or thymine) were incubated at in situ temperatures, in the dark, for 7 to 14 d. Concentrations of NH4+ and urea were monitored during the incubation period. Increases of NH4+ with time were observed in 8 out of 12 experiments with unsupplemented lake samples, and in a single trial with coastal water, but not with Charente estuary water. In some experiments, increases of urea concentrations were also observed. The addition of organic nitrogen compounds almost always led to NH4+ increases in samples from all locations; guanine, hypoxanthine, arginine and, in the case of Charente water, glucosamine gave rise to urea. The addition of nitrification inhibitors (40 PM) at the start of some experiments gave inconsistent results, but in some cases appeared to increase the concentrations of NH4+ with time. Taken together, the results of these experiments clearly indicate the potential in natural waters for degradation of DON pool constituents by indigenous bacteria and/or free dissolved enzymes to NH4+ or urea; these in turn can be effectively exploited by the ambient microbiota. The breakdown of DON with the concomitant release of readily available compounds such as NH4+ or urea could be an important process in the nitrogen nutrition of phytoplankton and bacteria.