Radio-labelled mercury, 203HgCl2, was added to water overlying sediment in freshwater model systems. The partitioning of 203Hg between water, sediment and air and the concentration of methyl 203Hg in the water did not differ significantly between systems with a water pH of 5.8+/-0.07 and 6.6+/-0.05. When the systems were made anaerobic and microbial activity was stimulated by the addition of tryptic soybroth (TSB), the level of methyl 203Hg in the water increased by two orders of magnitude. There was also an increase in the concentration of total 203Hg in the water, which was greater than the increase in methyl 203Hg, indicating release of both methyl 203Hg and other 203Hg species from sediment to water. In the systems that were kept aerobic, there was no increase in either total or methyl 203Hg in the water after stimulating the microbial activity with TSB. However, volatilization of 203Hg increased, which was not the case in the anaerobic systems.