Carbamazepine treatment of a patient with 48-hour rapid cycling led to a dampening of mood cycling, and prolonged rapid eye movement (REM) sleep latency. No effect on central alpha-receptors as measured by growth hormone (GH) secretion after clonidine stimulation or on spontaneous 48-hour GH secretion was observed. In 12 healthy subjects given 400 mg carbamazepine daily for a period of 5 days, improved sleep continuity and increased slow-wave sleep occurred with treatment. REM sleep percentage and REM latency remained uninfluenced, whereas REM density decreased. GH secretion after clonidine stimulation was not altered. Data from the single-case longitudinal study emphasize that carbamazepine is effective in treating rapid-cycling affective psychosis. Furthermore, neuroendocrine and sleep EEG data from the study in healthy subjects indicate a different profile of action for carbamazepine compared to most other antidepressants or antimanic drugs.