Nocturnal melatonin secretion and polysomnographic sleep patterns were investigated in ten patients with chronic primary insomnia (age 41.3 +/- 9.5 years) and in five healthy subject, (age 27.2 +/- 0.7 years) after either a single intravenous administration of 25 mg doxepin or placebo in a randomized, double blind, and cross-over setting. In the patient group a third session was performed after a three-week open oral treatment with 25 mg doxepin daily. The single-dose administration of doxepin did not affect plasma melatonin concentrations in either the patients on the healthy subjects. After three weeks of oral doxepin intake by the patients, the area under the curve of total nocturnal plasma melatonin concentration was significantly increased by 26% and the peak values were increased by 30%. Both after the single i.v. treatment as well as after long-term oral administration, doxepin also significantly improved sleep latency, total sleep time, and sleep efficiency in the insomniacs as well as the healthy subjects, whereas the nocturnal wake time was decreased. These findings indicate that this tricyclic antidepressant not only improves sleep and but also preserves the secretion of a hormone which is believed to play a special role in the circadian sleep-wake rhythm. Long-term doxepin treatment of chronic insomniac patients not only improves sleep but also restores nocturnal melatonin secretion in these patients.