Uptake, whole tissue, and subcellular distribution of methylmercury (MeHg) by rainbow trout, Salmo gairdneri, was studied during, and in three 2-wk intervals following, a single 24-h exposure to 14CH3HgCl and CH3203HgCl. Gel permeation chromatography of cytosol fractions was performed on select tissues. Gills contained approximately 10 times as much MeHg as any other tissue after 24-h exposure whereas concentrations were lowest for brain and skeletal muscle. In the 2-wk period following exposure, gill MeHg concentrations decreased to levels comparable with most other tissues; methylmercury in most other tissues increased and then decreased during the subsequent 4-wk period. Methylmercury levels in brain, skeletal muscle, and gonad were highest at 6 wk. During the 6-wk interval following MeHg exposure, the percent of mercury as inorganic mercury (Hg++) increased in gill, kidney, and liver and to a lesser extent in all other tissues except skeletal muscle. Total MeHg was greatest in cytosol fractions and usually made up from 50 to 80% of the total tissue mercury. In liver cytosol a methylmercury binding metallothionein-like species accounted for up to 40% of the total methylmercury bound. Inorganic mercury (Hg++), probably derived from demethylation of methylmercury, was associated with a metallothionein-like protein detected in gill tissue but no more than 6% of the total mercury in the soluble fraction was bound to this protein(s). Mercury binding to metallothionein-like proteins in kidney and splenic fractions was minimal in spite of relatively large amounts of mercury in the cytosol from these tissues. Metallothionein may act as a binding scavenger for methylmercury and inorganic mercury in fish. Key words: methylmercury, uptake by trout, tissue distribution, subcellular distribution, metallothionein, detoxification