Three days after the acute oral administration of methyl mercury (MeHg), a 27-60% increase in the total number of binding sites for [3H]diazepam was seen in the retina and different areas of the rat brain, with no change, except in the retina, in the apparent dissociation constant for its ligand. In contrast, MeHg failed to change [3H]spiroperidol and [3H]GABA binding in the same areas. Moreover, MeHg decreased cyclic GMP content in the cerebellar cortex. The various possible mechanisms involved in the action of MeHg on benzodiazepine binding are discussed.