The effect of 3 mg of melatonin on the rate of re-entrainment of plasma melatonin rhythm after an 11-h eastward flight was assessed. Eight subjects participated in the study, and underwent 24-h blood samplings once before the flight and twice after the flight. Subjects were exposed to natural zeitgeber outdoors and took 3 mg of malatonin at 20:00 h local time on the days when no blood sampling was done. Antidromic re-entrainment was dominant whereby melatonin administration in the evening promoted re-entrainment. Melatonin accelerated the rate of re-entrainment by 15 min per day and alleviated the jet lag symptoms.