Mercuric chloride was found to have a biphasic effect on the binding of the radiolabeled stimulant [3H]methylphenidate to membranes from a crude synaptoso-ma1 preparation of rat striatal tissue. Binding was enhanced at low concentrations of HgCl,, reaching a maximum of 62% above control values at 2.5 p M HgC1,. It was inhibited in a dose-dependent manner at concentrations greater than 5 pM HgCl,, with an IC,, of 7.2 pM. The increase in binding observed at the low concentrations of HgC1, was shown by Scatchard analysis to be due to an increase in the affinity of [3H]methylphenidate for its binding site on the dopamine transporter, while a decrease in both affinity and B, , , accompanied the reduction of [3H]methylphenidate binding observed at the higher concentrations of the inorganic mercury compound. The sodium salt of the organic mercurial p-chloromercuriphenylsulfonic acid also caused an increase in [3H]methylphenidate binding (159% above controls at 2,000 pM), followed by an immediate decrease in binding at higher concentrations of the reagent. Because both of these mercury-containing compounds have a high propensity for interacting with sulfhydryl groups, these data suggest that the cysteine residues in the dopamine transporter molecule may play an important role in the regulation of stimulant binding to the uptake complex. 0 1994 Wiley-Liss, Inc.