There are conflicting results regarding the effect of neuroleptic drugs on lithium ratio (LR) and methodological problems may be a reason for this. The effect of concurrent use of some psychotropic agents with lithium on the LR was studied in 113 Iranian patients with bipolar mood disorder during a prospective cohort in-patient study by using the new direct method of measuring the erythrocyte lithium concentration (ELC). Results revealed that patients taking a combination of lithium and neuroleptics had significantly lower LR and ELC values than those on lithium alone. Such differences were not found in patients who received carbamazepine, benzodiazepines or ECT concurrently with lithium. No relationship was found between the LR and lithium dosage or duration of treatment. The plasma lithium concentration may not significantly correlate with lithium dosage or duration. The effect of neuroleptics on the LR may be mediated through a stabilising effect on the cell membrane with a consequent reduction on lithium transport into the erythrocyte