To examine whether artificial light with the intensity commonly used for animal experimentation can mimic natural sunlight with respect to diurnal changes in serum melatonin, and to determine the minimum light intensity required to suppress nocturnal melatonin, serum melatonin profiles were examined in groups of female Japanese monkeys (Macaca fuscata fuscata). Under outdoor environment, light intensities at the level of the monkey's eyes varied during daytime (0900-1500 h) depending on weather conditions (minimum and maximum on particular experimental days: 170 lux at 0900 h on a rainy day and 9500 lux at 0900 h on a slightly cloudy day); under indoor environment, light was provided by ordinary fluorescent bulbs that resulted in intensities of 400-500 lux at the level of monkey's eyes. No difference was found in diurnal changes in serum melatonin concentrations regardless of weather or housing conditions: Serum melatonin remained low during daytime and increased during nighttime. Following exposure to light, irradiances of 10,000, 400-500, 100-140, 50-100, and 10-30 lux at midnight resulted in a rapid decrease in serum melatonin to daytime levels within 1 to 2 h. After the onset of dark, serum melatonin reverted to previous nighttime levels within 2 h. Exposure to a light irradiance of 2-5 lux, however, did not suppress nocturnal melatonin secretion. It is concluded that artificial light can mimic natural sunlight with respect to melatonin secretion in the female Japanese monkey, and that light of 10-30 lux irradiance was sufficient to suppress serum melatonin to near daytime levels.