The disposition of mercury in the blood, kidneys and liver was evaluated and compared in rats 5 min, 1 h, and 24 h after the intravenous administration of a 0.25 mumol/kg dose of inorganic mercury or a 0.25 mumol/kg dose of inorganic mercury plus a 0.5 mumol/kg dose of cysteine to determine the possible role of extracellular cysteine and complexes of cysteine and inorganic mercury in the renal uptake and transport of inorganic mercury. More inorganic mercury was present in the blood of the rats injected with inorganic mercury alone than in the blood of the rats injected simultaneously with both the inorganic mercury and cysteine during the first hour after injection. In addition, significantly more mercury was in the plasma fraction of blood in the rats injected with both inorganic mercury and cysteine than in the rats injected with inorganic mercury alone. These findings indicate that much of the mercury injected with cysteine was in some form of a complex that allowed the mercury to be cleared from the blood more readily and prevented the mercury from entering readily into the cellular components of blood. The renal concentration of mercury was significantly greater in the rats injected with both inorganic mercury and cysteine than in the rats injected with inorganic mercury alone 1 h, but not 24 h, after injection. This increased renal accumulation of mercury during the initial hour after injection was due mainly to enhanced uptake and/or retention of mercury in the renal cortex, although some of the enhanced accumulation of mercury also occurred in the outer stripe of the outer medulla during the first hour after injection. These data indicate that coadministration of a nontoxic dose of inorganic mercury with a twofold higher amount (in moles) of cysteine increases significantly the clearance of mercury from the blood and increases the accumulation of inorganic mercury in the renal cortex and outer stripe of the outer medulla during the initial 1 h after injection. In conclusion, the data in this study are consistent with the hypothesis that complexes of inorganic mercury and cysteine in the blood and/or ultrafiltrate probably play a role in the renal uptake of some of the mercury in blood after exposure to mercuric compounds.