The circadian rhythm of melatonin secretion was measured in 20 healthy males (20 - 32 yrs.) after intake of placebo or various doses of a special extract of Agnus castus (AC: 120 - 480 mg/die) for 14 days. A significant (p < 0.05) dose-dependent increase of the area under the melatonin secretion curve (AUC) was found. The pattern of circadian rhythm of melatonin secretion was, however, not influenced. It remains to be elucidated whether the AC-stimulated increase of melatonin secretion is suitable for treatment of sleep disturbances.