Total (T-Hg) and inorganic (I-Hg) mercury in blood and brain of female Macaca fascicularis monkeys, exposed to daily peroral doses of methyl mercury (MeHg; 50 micrograms Hg/kg body wt) for 6, 12, or 18 months, or to continuous iv infusion of HgCl2 (200 micrograms Hg/kg body wt) for 3 months, were determined. In normal weight monkeys (2.4-4.1 kg body wt) exposed to MeHg, steady state of T-Hg in blood (1.1 micrograms Hg/g) was reached in about 4 months. The elimination T1/2 in blood was 26 days. I-Hg constituted 7% of T-Hg in blood. The average concentration of MeHg in occipital pole and thalamus was about 3 micrograms Hg/g at 6 months and 4.5 micrograms Hg/g at 12-18 months. Accumulation in brain seemed to be biphasic. Following termination of 12 months exposure, elimination T1/2 for MeHg in brain was 35 days. I-Hg constituted about 9% of T-Hg in brain at 6-12 months, 18% at 18 months, and 74% at 6 months after termination of exposure. The I-Hg concentrations were somewhat higher in thalamus than in occipital pole. The elimination T1/2 for I-Hg was extremely long, on the order of years. Most likely, the I-Hg was formed by demethylation of MeHg in the brain. In monkeys exposed to HgCl2, blood levels of 0.6 micrograms I-Hg/g gave rise to brain I-Hg levels of about 0.1 micrograms/g only. In three heavy weight monkeys (5.0-6.1 kg body wt) exposed to MeHg, blood Hg increased to about 2 micrograms Hg/g, indicating a limited distribution of MeHg to fat. The Hg concentrations in brain (7-22 micrograms Hg/g) were considerably higher than those in normal weight monkeys, due to the high blood Hg levels in combination with a high brain-to-blood distribution ratio.