This study was designed to correlate autopsy findings with the effects on cage behavior, laboratory values, and mercury clearance of long‐term, low‐dose exposure of primates to methylmercury. Six rhesus monkeys were given daily methylmercury hydroxide (MeHg) orally in apple juice on a preplanned dosage schedule. Three were sacrificed while receiving MeHg (group I) and the other 3 were sacrificed 2–5 mo after cessation of MeHg administration (group II). Whole‐blood Hg levels (organic and inorganic) were assayed weekly, and major organ levels were assayed at autopsy. Whole‐blood Hg levels were maintained between 1 and 2 μg/ml when the monkeys were given a MeHg dose of 80–125 μg/kg·d for up to 1 yr. The Hg burden of the major organs appeared to be dose‐ and duration‐related. After periods of clearance (2.5–5 mo), intestinal wall Hg burden decreased to less than 1 μg/g, and the hepatic Hg burden was still between 1.12 and 2.37 μg/g. However, the kidneys had a higher concentration of Hg, ranging from 10.34 to 29.54 μg/g. Whenever there was a high concentration of Hg, significant ultrastructural changes were observed. In the kidneys there were intracytoplasmic vacuoles and electron‐dense inclusion bodies. In the small intestine of the animals cleared of mercury (group II), there were normal Paneth cells, as well as some degenerative cells characterized by dilation of endoplasmic reticulum and the presence of intracellular inclusion bodies. These findings suggest the long turnover time of Hg in these cell populations. During the period of study, weekly routine laboratory data including hematology, blood chemistry, and liver and kidney function tests did not reveal any significant changes.