The interaction between methyl mercury and ionizing radiation was investigated in a series of experiments using rats, hamsters, and squirrel monkeys to study the effects produced and possible mechanisms of action. Parameters evaluated included several measurements of behavior, brain electrical activity, lethality, blood-brain barrier permeability, neurotransmitter and mercury concentration in various brain areas, and brain histology. In some cases the effects of the co-insult were less than or at least no greater than at least one of the two insults applied alone. Possible mechanisms of action include opposite effects of the two insults on the blood-brain barrier, with radiation increasing permeability and methyl mercury decreasing it. Radiation may also elicit a proliferation of peroxisome-like organelles which protect against the effects of methyl mercury.