ABSTRACT.Lithium is a unique mood stabilizing agent with antimanic, antidepressant, and prophylactic properties for the management of recurrent affective illnesses. Lithium produces diffuse EEG slowing and paroxysmal activity which is more pronounced during chronic treatment than after a single acute dose and still more prominent during intoxication. EEG effects of lithium have been studied in patients with affective disorders and in normals as well as in adults and children with mainly consistent findings. The EEG is a more reliable indicator of neurotoxicity with lithium than are measures of plasma levels. Nevertheless there can be difficulty in discriminating between EEG alterations associated with therapeutic levels as opposed to toxicity. Quantitative methods of EEG analysis may offer more sensitive means of discrimination. There is an increased risk of lithium intoxication in patients with certain psychiatric diagnoses as well as in the elderly and in patients with underlying medical disorders. Toxicity is more likely when lithium is combined with other drugs, particularly neuroleptics. In these cases the EEG may disciminate between a neuroleptic malignant-like syndrome associated with high potency neuroleptics and lithium intoxication in combination with phenothiazines. Lithium has inconsistent effects in patients with epilepsy and may either enhance EEG abnormalities and aggravate seizures or conversely, stabilize epileptiform activity and reduce seizure incidence. However lithium is contraindicated in combination with electroconvulsive treatment. EEG studies are recommended in patients treated with lithium whenever there is a question of intoxication, as well as when clinical response is unsatisfactory or the patient's mental condition deteriorates. In these situations it may be necessary to obtain EEGs both on and off lithium to separate the relative importance of its therapeutic and toxic effects.