The effect of lithium carbonate on sleep was studied in seven patients with a manic-depressive or depressive illness. Drug administration had significant effects on the sleep pattern, especially on rapid eye movement (REM) and delta sleep. The REM sleep time, as well as the phasic REM elements, were reduced by lithium carbonate. Partial to complete reversal of the REM suppression occurred in the two patients in whom the dosage of the drug was reduced or discontinued. Delta sleep time was elevated with the use of lithium carbonate, and a higher dosage increased delta sleep to a greater extent than a moderate dosage in one patient.