Methylmercury (MeHg) readily crosses cell membrane barriers to reach its target tissue, the brain. Although it is generally assumed that this rapid transport is due to simple diffusion, recent studies have demonstrated that MeHg is transported as a hydrophilic complex, and possibly as an l -cysteine complex on the ubiquitous L-type large neutral amino acid transporters (LATs). To test this hypothesis, studies were carried out in Xenopus laevis oocytes expressing two of the major L-type carriers in humans, LAT1—4F2 heavy chain (4F2hc) and LAT2—4F2hc. Oocytes expressing LAT1—4F2hc or LAT2—4F2hc demonstrated enhanced uptake of [ 14 C]MeHg when administered as the l -cysteine or d , l -homocysteine complexes, but not when administered as the d -cysteine, N -acetyl- l -cysteine, penicillamine or GSH complexes. Kinetic analysis of transport indicated that the apparent affinities ( K m ) of MeHg— l -cysteine uptake by LAT1 and LAT2 (98±8 and 64±8 μ M respectively) were comparable with those for methionine (99±9 and 161±11 μ M), whereas the V max values were higher for MeHg— l -cysteine, indicating that it may be a better substrate than the endogenous amino acid. Uptake and efflux of [ 3 H]methionine and [ 14 C]MeHg— l -cysteine were trans -stimulated by leucine and phenylalanine, but not by glutamate, indicating that MeHg— l -cysteine is both a cis - and trans -substrate. In addition, [ 3 H]methionine efflux was trans -stimulated by leucine and phenylalanine even in the presence of an inwardly directed methionine gradient, demonstrating concentrative transport by both LAT1 and LAT2. The present results describe a major molecular mechanism by which MeHg is transported across cell membranes and indicate that metal complexes may form a novel class of substrates for amino acid carriers. These transport proteins may therefore participate in metal ion homoeostasis and toxicity.