We describe a compartmental pharmacokinetic model for methyl mercury and its metabolite mercuric mercury in humans. A tracer dose of 203Hg-labeled methyl mercury was administered iv to seven healthy young adult male volunteers. Blood samples were obtained periodically and urine and feces were collected throughout the 70 days of the study. The blood contained predominantly methyl mercury, while the excreta contained principally inorganic mercury. The behavior of both methyl mercury and inorganic mercury in the body was modeled with the simplest compartmental model which fit the data. This five-compartment model shows that inorganic mercury accumulates in the body and at longer times is the predominant form of mercury present. The biological half-life of methyl mercury in the body is 44 days and 1.6% of the body burden is lost each day by both metabolism and excretion. This rate of loss is 60% greater than that currently accepted (1.0% per day). Thus, the risk associated with dietary methyl mercury may have been overestimated.