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Published reports of toxic hallucinosis during treatment with methylphenidate are rare. Two cases are described, and it is suggested that methylphenidate-induced hallucinosis is more common than has been appreciated. Symptoms may be understood in terms of dose-related effects at pre- and postsynaptic receptors. Two other hypotheses that may clarify underlying mechanisms of action that produce toxic effects relate to (1) noradrenergic facilitation of transmission of information in the visual pathway, and (2) interactions among brain monoamine systems.